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PostgreSQL's cumulative statistics system supports collection and reporting of information about server activity. Presently, accesses to tables and indexes in both disk-block and individual-row terms are counted. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. If enabled, calls to user-defined functions and the total time spent in each one are counted as well.
PostgreSQL also supports reporting dynamic information about exactly what is going on in the system right now, such as the exact command currently being executed by other server processes, and which other connections exist in the system. This facility is independent of the cumulative statistics system.
27.2.1. Statistics Collection Configuration #Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. This is controlled by configuration parameters that are normally set in postgresql.conf. (See Chapter 19 for details about setting configuration parameters.)
The parameter track_activities enables monitoring of the current command being executed by any server process.
The parameter track_cost_delay_timing enables monitoring of cost-based vacuum delay.
The parameter track_counts controls whether cumulative statistics are collected about table and index accesses.
The parameter track_functions enables tracking of usage of user-defined functions.
The parameter track_io_timing enables monitoring of block read, write, extend, and fsync times.
The parameter track_wal_io_timing enables monitoring of WAL read, write and fsync times.
Normally these parameters are set in postgresql.conf so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the SET command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.)
Cumulative statistics are collected in shared memory. Every PostgreSQL process collects statistics locally, then updates the shared data at appropriate intervals. When a server, including a physical replica, shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. In contrast, when starting from an unclean shutdown (e.g., after an immediate shutdown, a server crash, starting from a base backup, and point-in-time recovery), all statistics counters are reset.
Several predefined views, listed in Table 27.1, are available to show the current state of the system. There are also several other views, listed in Table 27.2, available to show the accumulated statistics. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in Section 27.2.26.
When using the cumulative statistics views and functions to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process flushes out accumulated statistics to shared memory just before going idle, but not more frequently than once per PGSTAT_MIN_INTERVAL milliseconds (1 second unless altered while building the server); so a query or transaction still in progress does not affect the displayed totals and the displayed information lags behind actual activity. However, current-query information collected by track_activities is always up-to-date.
Another important point is that when a server process is asked to display any of the accumulated statistics, accessed values are cached until the end of its current transaction in the default configuration. So the statistics will show static information as long as you continue the current transaction. Similarly, information about the current queries of all sessions is collected when any such information is first requested within a transaction, and the same information will be displayed throughout the transaction. This is a feature, not a bug, because it allows you to perform several queries on the statistics and correlate the results without worrying that the numbers are changing underneath you. When analyzing statistics interactively, or with expensive queries, the time delta between accesses to individual statistics can lead to significant skew in the cached statistics. To minimize skew, stats_fetch_consistency can be set to snapshot, at the price of increased memory usage for caching not-needed statistics data. Conversely, if it's known that statistics are only accessed once, caching accessed statistics is unnecessary and can be avoided by setting stats_fetch_consistency to none. You can invoke pg_stat_clear_snapshot() to discard the current transaction's statistics snapshot or cached values (if any). The next use of statistical information will (when in snapshot mode) cause a new snapshot to be built or (when in cache mode) accessed statistics to be cached.
A transaction can also see its own statistics (not yet flushed out to the shared memory statistics) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. These numbers do not act as stated above; instead they update continuously throughout the transaction.
Some of the information in the dynamic statistics views shown in Table 27.1 is security restricted. Ordinary users can only see all the information about their own sessions (sessions belonging to a role that they are a member of). In rows about other sessions, many columns will be null. Note, however, that the existence of a session and its general properties such as its sessions user and database are visible to all users. Superusers and roles with privileges of built-in role pg_read_all_stats can see all the information about all sessions.
Table 27.1. Dynamic Statistics Views
View Name Descriptionpg_stat_activity One row per server process, showing information related to the current activity of that process, such as state and current query. See pg_stat_activity for details. pg_stat_replication One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See pg_stat_replication for details. pg_stat_wal_receiver Only one row, showing statistics about the WAL receiver from that receiver's connected server. See pg_stat_wal_receiver for details. pg_stat_recovery_prefetch Only one row, showing statistics about blocks prefetched during recovery. See pg_stat_recovery_prefetch for details. pg_stat_subscription At least one row per subscription, showing information about the subscription workers. See pg_stat_subscription for details. pg_stat_ssl One row per connection (regular and replication), showing information about SSL used on this connection. See pg_stat_ssl for details. pg_stat_gssapi One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. See pg_stat_gssapi for details. pg_stat_progress_analyze One row for each backend (including autovacuum worker processes) running ANALYZE, showing current progress. See Section 27.4.1. pg_stat_progress_create_index One row for each backend running CREATE INDEX or REINDEX, showing current progress. See Section 27.4.4. pg_stat_progress_vacuum One row for each backend (including autovacuum worker processes) running VACUUM, showing current progress. See Section 27.4.5. pg_stat_progress_cluster One row for each backend running CLUSTER or VACUUM FULL, showing current progress. See Section 27.4.2. pg_stat_progress_basebackup One row for each WAL sender process streaming a base backup, showing current progress. See Section 27.4.6. pg_stat_progress_copy One row for each backend running COPY, showing current progress. See Section 27.4.3.
Table 27.2. Collected Statistics Views
View Name Descriptionpg_stat_archiver One row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details. pg_stat_bgwriter One row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details. pg_stat_checkpointer One row only, showing statistics about the checkpointer process's activity. See pg_stat_checkpointer for details. pg_stat_database One row per database, showing database-wide statistics. See pg_stat_database for details. pg_stat_database_conflicts One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details. pg_stat_io One row for each combination of backend type, context, and target object containing cluster-wide I/O statistics. See pg_stat_io for details. pg_stat_replication_slots One row per replication slot, showing statistics about the replication slot's usage. See pg_stat_replication_slots for details. pg_stat_slru One row per SLRU, showing statistics of operations. See pg_stat_slru for details. pg_stat_subscription_stats One row per subscription, showing statistics about errors and conflicts. See pg_stat_subscription_stats for details. pg_stat_wal One row only, showing statistics about WAL activity. See pg_stat_wal for details. pg_stat_all_tables One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details. pg_stat_sys_tables Same as pg_stat_all_tables, except that only system tables are shown. pg_stat_user_tables Same as pg_stat_all_tables, except that only user tables are shown. pg_stat_xact_all_tables Similar to pg_stat_all_tables, but counts actions taken so far within the current transaction (which are not yet included in pg_stat_all_tables and related views). The columns for numbers of live and dead rows and vacuum and analyze actions are not present in this view. pg_stat_xact_sys_tables Same as pg_stat_xact_all_tables, except that only system tables are shown. pg_stat_xact_user_tables Same as pg_stat_xact_all_tables, except that only user tables are shown. pg_stat_all_indexes One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details. pg_stat_sys_indexes Same as pg_stat_all_indexes, except that only indexes on system tables are shown. pg_stat_user_indexes Same as pg_stat_all_indexes, except that only indexes on user tables are shown. pg_stat_user_functions One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details. pg_stat_xact_user_functions Similar to pg_stat_user_functions, but counts only calls during the current transaction (which are not yet included in pg_stat_user_functions). pg_statio_all_tables One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details. pg_statio_sys_tables Same as pg_statio_all_tables, except that only system tables are shown. pg_statio_user_tables Same as pg_statio_all_tables, except that only user tables are shown. pg_statio_all_indexes One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details. pg_statio_sys_indexes Same as pg_statio_all_indexes, except that only indexes on system tables are shown. pg_statio_user_indexes Same as pg_statio_all_indexes, except that only indexes on user tables are shown. pg_statio_all_sequences One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details. pg_statio_sys_sequences Same as pg_statio_all_sequences, except that only system sequences are shown. (Presently, no system sequences are defined, so this view is always empty.) pg_statio_user_sequences Same as pg_statio_all_sequences, except that only user sequences are shown.
The per-index statistics are particularly useful to determine which indexes are being used and how effective they are.
The pg_stat_io and pg_statio_ set of views are useful for determining the effectiveness of the buffer cache. They can be used to calculate a cache hit ratio. Note that while PostgreSQL's I/O statistics capture most instances in which the kernel was invoked in order to perform I/O, they do not differentiate between data which had to be fetched from disk and that which already resided in the kernel page cache. Users are advised to use the PostgreSQL statistics views in combination with operating system utilities for a more complete picture of their database's I/O performance.
pg_stat_activity #
The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process.
Table 27.3. pg_stat_activity View
Column Type
Description
datid oid
OID of the database this backend is connected to
datname name
Name of the database this backend is connected to
pid integer
Process ID of this backend
leader_pid integer
Process ID of the parallel group leader if this process is a parallel query worker, or process ID of the leader apply worker if this process is a parallel apply worker. NULL indicates that this process is a parallel group leader or leader apply worker, or does not participate in any parallel operation.
usesysid oid
OID of the user logged into this backend
usename name
Name of the user logged into this backend
application_name text
Name of the application that is connected to this backend
client_addr inet
IP address of the client connected to this backend. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum.
client_hostname text
Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled.
client_port integer
TCP port number that the client is using for communication with this backend, or -1 if a Unix socket is used. If this field is null, it indicates that this is an internal server process.
backend_start timestamp with time zone
Time when this process was started. For client backends, this is the time the client connected to the server.
xact_start timestamp with time zone
Time when this process' current transaction was started, or null if no transaction is active. If the current query is the first of its transaction, this column is equal to the query_start column.
query_start timestamp with time zone
Time when the currently active query was started, or if state is not active, when the last query was started
state_change timestamp with time zone
Time when the state was last changed
wait_event_type text
The type of event for which the backend is waiting, if any; otherwise NULL. See Table 27.4.
wait_event text
Wait event name if backend is currently waiting, otherwise NULL. See Table 27.5 through Table 27.13.
state text
Current overall state of this backend. Possible values are:
starting: The backend is in initial startup. Client authentication is performed during this phase.
active: The backend is executing a query.
idle: The backend is waiting for a new client command.
idle in transaction: The backend is in a transaction, but is not currently executing a query.
idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error.
fastpath function call: The backend is executing a fast-path function.
disabled: This state is reported if track_activities is disabled in this backend.
backend_xid xid
Top-level transaction identifier of this backend, if any; see Section 67.1.
backend_xmin xid
The current backend's xmin horizon.
query_id bigint
Identifier of this backend's most recent query. If state is active this field shows the identifier of the currently executing query. In all other states, it shows the identifier of last query that was executed. Query identifiers are not computed by default so this field will be null unless compute_query_id parameter is enabled or a third-party module that computes query identifiers is configured.
query text
Text of this backend's most recent query. If state is active this field shows the currently executing query. In all other states, it shows the last query that was executed. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter track_activity_query_size.
backend_type text
Type of current backend. Possible types are autovacuum launcher, autovacuum worker, logical replication launcher, logical replication worker, parallel worker, background writer, client backend, checkpointer, archiver, standalone backend, startup, walreceiver, walsender, walwriter and walsummarizer. In addition, background workers registered by extensions may have additional types.
The wait_event and state columns are independent. If a backend is in the active state, it may or may not be waiting on some event. If the state is active and wait_event is non-null, it means that a query is being executed, but is being blocked somewhere in the system. To keep the reporting overhead low, the system does not attempt to synchronize different aspects of activity data for a backend. As a result, ephemeral discrepancies may exist between the view's columns.
Table 27.4. Wait Event Types
Wait Event Type DescriptionActivity The server process is idle. This event type indicates a process waiting for activity in its main processing loop. wait_event will identify the specific wait point; see Table 27.5. BufferPin The server process is waiting for exclusive access to a data buffer. Buffer pin waits can be protracted if another process holds an open cursor that last read data from the buffer in question. See Table 27.6. Client The server process is waiting for activity on a socket connected to a user application. Thus, the server expects something to happen that is independent of its internal processes. wait_event will identify the specific wait point; see Table 27.7. Extension The server process is waiting for some condition defined by an extension module. See Table 27.8. InjectionPoint The server process is waiting for an injection point to reach an outcome defined in a test. See Section 36.10.14 for more details. This type has no predefined wait points. IO The server process is waiting for an I/O operation to complete. wait_event will identify the specific wait point; see Table 27.9. IPC The server process is waiting for some interaction with another server process. wait_event will identify the specific wait point; see Table 27.10. Lock The server process is waiting for a heavyweight lock. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. wait_event will identify the type of lock awaited; see Table 27.11. LWLock The server process is waiting for a lightweight lock. Most such locks protect a particular data structure in shared memory. wait_event will contain a name identifying the purpose of the lightweight lock. (Some locks have specific names; others are part of a group of locks each with a similar purpose.) See Table 27.12. Timeout The server process is waiting for a timeout to expire. wait_event will identify the specific wait point; see Table 27.13.
Table 27.5. Wait Events of Type Activity
Activity Wait Event Description ArchiverMain Waiting in main loop of archiver process. AutovacuumMain Waiting in main loop of autovacuum launcher process. BgwriterHibernate Waiting in background writer process, hibernating. BgwriterMain Waiting in main loop of background writer process. CheckpointerMain Waiting in main loop of checkpointer process. CheckpointerShutdown Waiting for checkpointer process to be terminated. IoWorkerMain Waiting in main loop of IO Worker process. LogicalApplyMain Waiting in main loop of logical replication apply process. LogicalLauncherMain Waiting in main loop of logical replication launcher process. LogicalParallelApplyMain Waiting in main loop of logical replication parallel apply process. RecoveryWalStream Waiting in main loop of startup process for WAL to arrive, during streaming recovery. ReplicationSlotsyncMain Waiting in main loop of slot sync worker. ReplicationSlotsyncShutdown Waiting for slot sync worker to shut down. SysloggerMain Waiting in main loop of syslogger process. WalReceiverMain Waiting in main loop of WAL receiver process. WalSenderMain Waiting in main loop of WAL sender process. WalSummarizerWal Waiting in WAL summarizer for more WAL to be generated. WalWriterMain Waiting in main loop of WAL writer process.
Table 27.6. Wait Events of Type Bufferpin
BufferPin Wait Event Description BufferPin Waiting to acquire an exclusive pin on a buffer.
Table 27.7. Wait Events of Type Client
Client Wait Event Description ClientRead Waiting to read data from the client. ClientWrite Waiting to write data to the client. GssOpenServer Waiting to read data from the client while establishing a GSSAPI session. LibpqwalreceiverConnect Waiting in WAL receiver to establish connection to remote server. LibpqwalreceiverReceive Waiting in WAL receiver to receive data from remote server. SslOpenServer Waiting for SSL while attempting connection. WaitForStandbyConfirmation Waiting for WAL to be received and flushed by the physical standby. WalSenderWaitForWal Waiting for WAL to be flushed in WAL sender process. WalSenderWriteData Waiting for any activity when processing replies from WAL receiver in WAL sender process.
Table 27.8. Wait Events of Type Extension
Extension Wait Event Description Extension Waiting in an extension.
Table 27.9. Wait Events of Type Io
IO Wait Event Description AioIoCompletion Waiting for another process to complete IO. AioIoUringExecution Waiting for IO execution via io_uring. AioIoUringSubmit Waiting for IO submission via io_uring. BasebackupRead Waiting for base backup to read from a file. BasebackupSync Waiting for data written by a base backup to reach durable storage. BasebackupWrite Waiting for base backup to write to a file. BuffileRead Waiting for a read from a buffered file. BuffileTruncate Waiting for a buffered file to be truncated. BuffileWrite Waiting for a write to a buffered file. ControlFileRead Waiting for a read from the pg_control file. ControlFileSync Waiting for the pg_control file to reach durable storage. ControlFileSyncUpdate Waiting for an update to the pg_control file to reach durable storage. ControlFileWrite Waiting for a write to the pg_control file. ControlFileWriteUpdate Waiting for a write to update the pg_control file. CopyFileCopy Waiting for a file copy operation. CopyFileRead Waiting for a read during a file copy operation. CopyFileWrite Waiting for a write during a file copy operation. DataFileExtend Waiting for a relation data file to be extended. DataFileFlush Waiting for a relation data file to reach durable storage. DataFileImmediateSync Waiting for an immediate synchronization of a relation data file to durable storage. DataFilePrefetch Waiting for an asynchronous prefetch from a relation data file. DataFileRead Waiting for a read from a relation data file. DataFileSync Waiting for changes to a relation data file to reach durable storage. DataFileTruncate Waiting for a relation data file to be truncated. DataFileWrite Waiting for a write to a relation data file. DsmAllocate Waiting for a dynamic shared memory segment to be allocated. DsmFillZeroWrite Waiting to fill a dynamic shared memory backing file with zeroes. LockFileAddtodatadirRead Waiting for a read while adding a line to the data directory lock file. LockFileAddtodatadirSync Waiting for data to reach durable storage while adding a line to the data directory lock file. LockFileAddtodatadirWrite Waiting for a write while adding a line to the data directory lock file. LockFileCreateRead Waiting to read while creating the data directory lock file. LockFileCreateSync Waiting for data to reach durable storage while creating the data directory lock file. LockFileCreateWrite Waiting for a write while creating the data directory lock file. LockFileRecheckdatadirRead Waiting for a read during recheck of the data directory lock file. LogicalRewriteCheckpointSync Waiting for logical rewrite mappings to reach durable storage during a checkpoint. LogicalRewriteMappingSync Waiting for mapping data to reach durable storage during a logical rewrite. LogicalRewriteMappingWrite Waiting for a write of mapping data during a logical rewrite. LogicalRewriteSync Waiting for logical rewrite mappings to reach durable storage. LogicalRewriteTruncate Waiting for truncate of mapping data during a logical rewrite. LogicalRewriteWrite Waiting for a write of logical rewrite mappings. RelationMapRead Waiting for a read of the relation map file. RelationMapReplace Waiting for durable replacement of a relation map file. RelationMapWrite Waiting for a write to the relation map file. ReorderBufferRead Waiting for a read during reorder buffer management. ReorderBufferWrite Waiting for a write during reorder buffer management. ReorderLogicalMappingRead Waiting for a read of a logical mapping during reorder buffer management. ReplicationSlotRead Waiting for a read from a replication slot control file. ReplicationSlotRestoreSync Waiting for a replication slot control file to reach durable storage while restoring it to memory. ReplicationSlotSync Waiting for a replication slot control file to reach durable storage. ReplicationSlotWrite Waiting for a write to a replication slot control file. SlruFlushSync Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown. SlruRead Waiting for a read of an SLRU page. SlruSync Waiting for SLRU data to reach durable storage following a page write. SlruWrite Waiting for a write of an SLRU page. SnapbuildRead Waiting for a read of a serialized historical catalog snapshot. SnapbuildSync Waiting for a serialized historical catalog snapshot to reach durable storage. SnapbuildWrite Waiting for a write of a serialized historical catalog snapshot. TimelineHistoryFileSync Waiting for a timeline history file received via streaming replication to reach durable storage. TimelineHistoryFileWrite Waiting for a write of a timeline history file received via streaming replication. TimelineHistoryRead Waiting for a read of a timeline history file. TimelineHistorySync Waiting for a newly created timeline history file to reach durable storage. TimelineHistoryWrite Waiting for a write of a newly created timeline history file. TwophaseFileRead Waiting for a read of a two phase state file. TwophaseFileSync Waiting for a two phase state file to reach durable storage. TwophaseFileWrite Waiting for a write of a two phase state file. VersionFileSync Waiting for the version file to reach durable storage while creating a database. VersionFileWrite Waiting for the version file to be written while creating a database. WalsenderTimelineHistoryRead Waiting for a read from a timeline history file during a walsender timeline command. WalBootstrapSync Waiting for WAL to reach durable storage during bootstrapping. WalBootstrapWrite Waiting for a write of a WAL page during bootstrapping. WalCopyRead Waiting for a read when creating a new WAL segment by copying an existing one. WalCopySync Waiting for a new WAL segment created by copying an existing one to reach durable storage. WalCopyWrite Waiting for a write when creating a new WAL segment by copying an existing one. WalInitSync Waiting for a newly initialized WAL file to reach durable storage. WalInitWrite Waiting for a write while initializing a new WAL file. WalRead Waiting for a read from a WAL file. WalSummaryRead Waiting for a read from a WAL summary file. WalSummaryWrite Waiting for a write to a WAL summary file. WalSync Waiting for a WAL file to reach durable storage. WalSyncMethodAssign Waiting for data to reach durable storage while assigning a new WAL sync method. WalWrite Waiting for a write to a WAL file.
Table 27.10. Wait Events of Type Ipc
IPC Wait Event Description AppendReady Waiting for subplan nodes of an Append plan node to be ready. ArchiveCleanupCommand Waiting for archive_cleanup_command to complete. ArchiveCommand Waiting for archive_command to complete. BackendTermination Waiting for the termination of another backend. BackupWaitWalArchive Waiting for WAL files required for a backup to be successfully archived. BgworkerShutdown Waiting for background worker to shut down. BgworkerStartup Waiting for background worker to start up. BtreePage Waiting for the page number needed to continue a parallel B-tree scan to become available. BufferIo Waiting for buffer I/O to complete. CheckpointDelayComplete Waiting for a backend that blocks a checkpoint from completing. CheckpointDelayStart Waiting for a backend that blocks a checkpoint from starting. CheckpointDone Waiting for a checkpoint to complete. CheckpointStart Waiting for a checkpoint to start. ExecuteGather Waiting for activity from a child process while executing a Gather plan node. HashBatchAllocate Waiting for an elected Parallel Hash participant to allocate a hash table. HashBatchElect Waiting to elect a Parallel Hash participant to allocate a hash table. HashBatchLoad Waiting for other Parallel Hash participants to finish loading a hash table. HashBuildAllocate Waiting for an elected Parallel Hash participant to allocate the initial hash table. HashBuildElect Waiting to elect a Parallel Hash participant to allocate the initial hash table. HashBuildHashInner Waiting for other Parallel Hash participants to finish hashing the inner relation. HashBuildHashOuter Waiting for other Parallel Hash participants to finish partitioning the outer relation. HashGrowBatchesDecide Waiting to elect a Parallel Hash participant to decide on future batch growth. HashGrowBatchesElect Waiting to elect a Parallel Hash participant to allocate more batches. HashGrowBatchesFinish Waiting for an elected Parallel Hash participant to decide on future batch growth. HashGrowBatchesReallocate Waiting for an elected Parallel Hash participant to allocate more batches. HashGrowBatchesRepartition Waiting for other Parallel Hash participants to finish repartitioning. HashGrowBucketsElect Waiting to elect a Parallel Hash participant to allocate more buckets. HashGrowBucketsReallocate Waiting for an elected Parallel Hash participant to finish allocating more buckets. HashGrowBucketsReinsert Waiting for other Parallel Hash participants to finish inserting tuples into new buckets. LogicalApplySendData Waiting for a logical replication leader apply process to send data to a parallel apply process. LogicalParallelApplyStateChange Waiting for a logical replication parallel apply process to change state. LogicalSyncData Waiting for a logical replication remote server to send data for initial table synchronization. LogicalSyncStateChange Waiting for a logical replication remote server to change state. MessageQueueInternal Waiting for another process to be attached to a shared message queue. MessageQueuePutMessage Waiting to write a protocol message to a shared message queue. MessageQueueReceive Waiting to receive bytes from a shared message queue. MessageQueueSend Waiting to send bytes to a shared message queue. MultixactCreation Waiting for a multixact creation to complete. ParallelBitmapScan Waiting for parallel bitmap scan to become initialized. ParallelCreateIndexScan Waiting for parallel CREATE INDEX workers to finish heap scan. ParallelFinish Waiting for parallel workers to finish computing. ProcarrayGroupUpdate Waiting for the group leader to clear the transaction ID at transaction end. ProcSignalBarrier Waiting for a barrier event to be processed by all backends. Promote Waiting for standby promotion. RecoveryConflictSnapshot Waiting for recovery conflict resolution for a vacuum cleanup. RecoveryConflictTablespace Waiting for recovery conflict resolution for dropping a tablespace. RecoveryEndCommand Waiting for recovery_end_command to complete. RecoveryPause Waiting for recovery to be resumed. ReplicationOriginDrop Waiting for a replication origin to become inactive so it can be dropped. ReplicationSlotDrop Waiting for a replication slot to become inactive so it can be dropped. RestoreCommand Waiting for restore_command to complete. SafeSnapshot Waiting to obtain a valid snapshot for a READ ONLY DEFERRABLE transaction. SyncRep Waiting for confirmation from a remote server during synchronous replication. WalBufferInit Waiting on WAL buffer to be initialized. WalReceiverExit Waiting for the WAL receiver to exit. WalReceiverWaitStart Waiting for startup process to send initial data for streaming replication. WalSummaryReady Waiting for a new WAL summary to be generated. XactGroupUpdate Waiting for the group leader to update transaction status at transaction end.
Table 27.11. Wait Events of Type Lock
Lock Wait Event Description advisory Waiting to acquire an advisory user lock. applytransaction Waiting to acquire a lock on a remote transaction being applied by a logical replication subscriber. extend Waiting to extend a relation. frozenid Waiting to update pg_database.datfrozenxid and pg_database.datminmxid. object Waiting to acquire a lock on a non-relation database object. page Waiting to acquire a lock on a page of a relation. relation Waiting to acquire a lock on a relation. spectoken Waiting to acquire a speculative insertion lock. transactionid Waiting for a transaction to finish. tuple Waiting to acquire a lock on a tuple. userlock Waiting to acquire a user lock. virtualxid Waiting to acquire a virtual transaction ID lock; see Section 67.1.
Table 27.12. Wait Events of Type Lwlock
LWLock Wait Event Description AddinShmemInit Waiting to manage an extension's space allocation in shared memory. AioUringCompletion Waiting for another process to complete IO via io_uring. AioWorkerSubmissionQueue Waiting to access AIO worker submission queue. AutoFile Waiting to update the postgresql.auto.conf file. Autovacuum Waiting to read or update the current state of autovacuum workers. AutovacuumSchedule Waiting to ensure that a table selected for autovacuum still needs vacuuming. BackgroundWorker Waiting to read or update background worker state. BtreeVacuum Waiting to read or update vacuum-related information for a B-tree index. BufferContent Waiting to access a data page in memory. BufferMapping Waiting to associate a data block with a buffer in the buffer pool. CheckpointerComm Waiting to manage fsync requests. CommitTs Waiting to read or update the last value set for a transaction commit timestamp. CommitTsBuffer Waiting for I/O on a commit timestamp SLRU buffer. CommitTsSLRU Waiting to access the commit timestamp SLRU cache. ControlFile Waiting to read or update the pg_control file or create a new WAL file. DSMRegistry Waiting to read or update the dynamic shared memory registry. DSMRegistryDSA Waiting to access dynamic shared memory registry's dynamic shared memory allocator. DSMRegistryHash Waiting to access dynamic shared memory registry's shared hash table. DynamicSharedMemoryControl Waiting to read or update dynamic shared memory allocation information. InjectionPoint Waiting to read or update information related to injection points. LockFastPath Waiting to read or update a process' fast-path lock information. LockManager Waiting to read or update information about “heavyweight” locks. LogicalRepLauncherDSA Waiting to access logical replication launcher's dynamic shared memory allocator. LogicalRepLauncherHash Waiting to access logical replication launcher's shared hash table. LogicalRepWorker Waiting to read or update the state of logical replication workers. MultiXactGen Waiting to read or update shared multixact state. MultiXactMemberBuffer Waiting for I/O on a multixact member SLRU buffer. MultiXactMemberSLRU Waiting to access the multixact member SLRU cache. MultiXactOffsetBuffer Waiting for I/O on a multixact offset SLRU buffer. MultiXactOffsetSLRU Waiting to access the multixact offset SLRU cache. MultiXactTruncation Waiting to read or truncate multixact information. NotifyBuffer Waiting for I/O on a NOTIFY message SLRU buffer. NotifyQueue Waiting to read or update NOTIFY messages. NotifyQueueTail Waiting to update limit on NOTIFY message storage. NotifySLRU Waiting to access the NOTIFY message SLRU cache. OidGen Waiting to allocate a new OID. ParallelAppend Waiting to choose the next subplan during Parallel Append plan execution. ParallelBtreeScan Waiting to synchronize workers during Parallel B-tree scan plan execution. ParallelHashJoin Waiting to synchronize workers during Parallel Hash Join plan execution. ParallelQueryDSA Waiting for parallel query dynamic shared memory allocation. ParallelVacuumDSA Waiting for parallel vacuum dynamic shared memory allocation. PerSessionDSA Waiting for parallel query dynamic shared memory allocation. PerSessionRecordType Waiting to access a parallel query's information about composite types. PerSessionRecordTypmod Waiting to access a parallel query's information about type modifiers that identify anonymous record types. PerXactPredicateList Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query. PgStatsData Waiting for shared memory stats data access. PgStatsDSA Waiting for stats dynamic shared memory allocator access. PgStatsHash Waiting for stats shared memory hash table access. PredicateLockManager Waiting to access predicate lock information used by serializable transactions. ProcArray Waiting to access the shared per-process data structures (typically, to get a snapshot or report a session's transaction ID). RelationMapping Waiting to read or update a pg_filenode.map file (used to track the filenode assignments of certain system catalogs). RelCacheInit Waiting to read or update a pg_internal.init relation cache initialization file. ReplicationOrigin Waiting to create, drop or use a replication origin. ReplicationOriginState Waiting to read or update the progress of one replication origin. ReplicationSlotAllocation Waiting to allocate or free a replication slot. ReplicationSlotControl Waiting to read or update replication slot state. ReplicationSlotIO Waiting for I/O on a replication slot. SerialBuffer Waiting for I/O on a serializable transaction conflict SLRU buffer. SerialControl Waiting to read or update shared pg_serial state. SerializableFinishedList Waiting to access the list of finished serializable transactions. SerializablePredicateList Waiting to access the list of predicate locks held by serializable transactions. SerializableXactHash Waiting to read or update information about serializable transactions. SerialSLRU Waiting to access the serializable transaction conflict SLRU cache. SharedTidBitmap Waiting to access a shared TID bitmap during a parallel bitmap index scan. SharedTupleStore Waiting to access a shared tuple store during parallel query. ShmemIndex Waiting to find or allocate space in shared memory. SInvalRead Waiting to retrieve messages from the shared catalog invalidation queue. SInvalWrite Waiting to add a message to the shared catalog invalidation queue. SubtransBuffer Waiting for I/O on a sub-transaction SLRU buffer. SubtransSLRU Waiting to access the sub-transaction SLRU cache. SyncRep Waiting to read or update information about the state of synchronous replication. SyncScan Waiting to select the starting location of a synchronized table scan. TablespaceCreate Waiting to create or drop a tablespace. TwoPhaseState Waiting to read or update the state of prepared transactions. WaitEventCustom Waiting to read or update custom wait events information. WALInsert Waiting to insert WAL data into a memory buffer. WALSummarizer Waiting to read or update WAL summarization state. WALWrite Waiting for WAL buffers to be written to disk. WrapLimitsVacuum Waiting to update limits on transaction id and multixact consumption. XactBuffer Waiting for I/O on a transaction status SLRU buffer. XactSLRU Waiting to access the transaction status SLRU cache. XactTruncation Waiting to execute pg_xact_status or update the oldest transaction ID available to it. XidGen Waiting to allocate a new transaction ID.
Table 27.13. Wait Events of Type Timeout
Timeout Wait Event Description BaseBackupThrottle Waiting during base backup when throttling activity. CheckpointWriteDelay Waiting between writes while performing a checkpoint. PgSleep Waiting due to a call to pg_sleep or a sibling function. RecoveryApplyDelay Waiting to apply WAL during recovery because of a delay setting. RecoveryRetrieveRetryInterval Waiting during recovery when WAL data is not available from any source (pg_wal, archive or stream). RegisterSyncRequest Waiting while sending synchronization requests to the checkpointer, because the request queue is full. SpinDelay Waiting while acquiring a contended spinlock. VacuumDelay Waiting in a cost-based vacuum delay point. VacuumTruncate Waiting to acquire an exclusive lock to truncate off any empty pages at the end of a table vacuumed. WalSummarizerError Waiting after a WAL summarizer error.
Here are examples of how wait events can be viewed:
SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL; pid | wait_event_type | wait_event ------+-----------------+------------ 2540 | Lock | relation 6644 | LWLock | ProcArray (2 rows)
SELECT a.pid, a.wait_event, w.description
FROM pg_stat_activity a JOIN
pg_wait_events w ON (a.wait_event_type = w.type AND
a.wait_event = w.name)
WHERE a.wait_event is NOT NULL and a.state = 'active';
-[ RECORD 1 ]------------------------------------------------------------------
pid | 686674
wait_event | WALInitSync
description | Waiting for a newly initialized WAL file to reach durable storage
Note
Extensions can add Extension, InjectionPoint, and LWLock events to the lists shown in Table 27.8 and Table 27.12. In some cases, the name of an LWLock assigned by an extension will not be available in all server processes. It might be reported as just “extension” rather than the extension-assigned name.
pg_stat_replication #
The pg_stat_replication view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. Only directly connected standbys are listed; no information is available about downstream standby servers.
Table 27.14. pg_stat_replication View
Column Type
Description
pid integer
Process ID of a WAL sender process
usesysid oid
OID of the user logged into this WAL sender process
usename name
Name of the user logged into this WAL sender process
application_name text
Name of the application that is connected to this WAL sender
client_addr inet
IP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine.
client_hostname text
Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled.
client_port integer
TCP port number that the client is using for communication with this WAL sender, or -1 if a Unix socket is used
backend_start timestamp with time zone
Time when this process was started, i.e., when the client connected to this WAL sender
backend_xmin xid
This standby's xmin horizon reported by hot_standby_feedback.
state text
Current WAL sender state. Possible values are:
startup: This WAL sender is starting up.
catchup: This WAL sender's connected standby is catching up with the primary.
streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary.
backup: This WAL sender is sending a backup.
stopping: This WAL sender is stopping.
sent_lsn pg_lsn
Last write-ahead log location sent on this connection
write_lsn pg_lsn
Last write-ahead log location written to disk by this standby server
flush_lsn pg_lsn
Last write-ahead log location flushed to disk by this standby server
replay_lsn pg_lsn
Last write-ahead log location replayed into the database on this standby server
write_lag interval
Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written it (but not yet flushed it or applied it). This can be used to gauge the delay that synchronous_commit level remote_write incurred while committing if this server was configured as a synchronous standby.
flush_lag interval
Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written and flushed it (but not yet applied it). This can be used to gauge the delay that synchronous_commit level on incurred while committing if this server was configured as a synchronous standby.
replay_lag interval
Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. This can be used to gauge the delay that synchronous_commit level remote_apply incurred while committing if this server was configured as a synchronous standby.
sync_priority integer
Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. This has no effect in a quorum-based synchronous replication.
sync_state text
Synchronous state of this standby server. Possible values are:
async: This standby server is asynchronous.
potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails.
sync: This standby server is synchronous.
quorum: This standby server is considered as a candidate for quorum standbys.
reply_time timestamp with time zone
Send time of last reply message received from standby server
The lag times reported in the pg_stat_replication view are measurements of the time taken for recent WAL to be written, flushed and replayed and for the sender to know about it. These times represent the commit delay that was (or would have been) introduced by each synchronous commit level, if the remote server was configured as a synchronous standby. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. If the standby server has entirely caught up with the sending server and there is no more WAL activity, the most recently measured lag times will continue to be displayed for a short time and then show NULL.
Lag times work automatically for physical replication. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag.
NoteThe reported lag times are not predictions of how long it will take for the standby to catch up with the sending server assuming the current rate of replay. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. In particular, when the standby has caught up completely, pg_stat_replication shows the time taken to write, flush and replay the most recent reported WAL location rather than zero as some users might expect. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. To reduce confusion for users expecting a different model of lag, the lag columns revert to NULL after a short time on a fully replayed idle system. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value.
pg_stat_replication_slots #
The pg_stat_replication_slots view will contain one row per logical replication slot, showing statistics about its usage.
Table 27.15. pg_stat_replication_slots View
Column Type
Description
slot_name text
A unique, cluster-wide identifier for the replication slot
spill_txns bigint
Number of transactions spilled to disk once the memory used by logical decoding to decode changes from WAL has exceeded logical_decoding_work_mem. The counter gets incremented for both top-level transactions and subtransactions.
spill_count bigint
Number of times transactions were spilled to disk while decoding changes from WAL for this slot. This counter is incremented each time a transaction is spilled, and the same transaction may be spilled multiple times.
spill_bytes bigint
Amount of decoded transaction data spilled to disk while performing decoding of changes from WAL for this slot. This and other spill counters can be used to gauge the I/O which occurred during logical decoding and allow tuning logical_decoding_work_mem.
stream_txns bigint
Number of in-progress transactions streamed to the decoding output plugin after the memory used by logical decoding to decode changes from WAL for this slot has exceeded logical_decoding_work_mem. Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions.
stream_countbigint
Number of times in-progress transactions were streamed to the decoding output plugin while decoding changes from WAL for this slot. This counter is incremented each time a transaction is streamed, and the same transaction may be streamed multiple times.
stream_bytesbigint
Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. This and other streaming counters for this slot can be used to tune logical_decoding_work_mem.
total_txns bigint
Number of decoded transactions sent to the decoding output plugin for this slot. This counts top-level transactions only, and is not incremented for subtransactions. Note that this includes the transactions that are streamed and/or spilled.
total_bytesbigint
Amount of transaction data decoded for sending transactions to the decoding output plugin while decoding changes from WAL for this slot. Note that this includes data that is streamed and/or spilled.
stats_reset timestamp with time zone
Time at which these statistics were last reset
pg_stat_wal_receiver #
The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server.
Table 27.16. pg_stat_wal_receiver View
Column Type
Description
pid integer
Process ID of the WAL receiver process
status text
Activity status of the WAL receiver process
receive_start_lsn pg_lsn
First write-ahead log location used when WAL receiver is started
receive_start_tli integer
First timeline number used when WAL receiver is started
written_lsn pg_lsn
Last write-ahead log location already received and written to disk, but not flushed. This should not be used for data integrity checks.
flushed_lsn pg_lsn
Last write-ahead log location already received and flushed to disk, the initial value of this field being the first log location used when WAL receiver is started
received_tli integer
Timeline number of last write-ahead log location received and flushed to disk, the initial value of this field being the timeline number of the first log location used when WAL receiver is started
last_msg_send_time timestamp with time zone
Send time of last message received from origin WAL sender
last_msg_receipt_time timestamp with time zone
Receipt time of last message received from origin WAL sender
latest_end_lsn pg_lsn
Last write-ahead log location reported to origin WAL sender
latest_end_time timestamp with time zone
Time of last write-ahead log location reported to origin WAL sender
slot_name text
Replication slot name used by this WAL receiver
sender_host text
Host of the PostgreSQL instance this WAL receiver is connected to. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. (The path case can be distinguished because it will always be an absolute path, beginning with /.)
sender_port integer
Port number of the PostgreSQL instance this WAL receiver is connected to.
conninfo text
Connection string used by this WAL receiver, with security-sensitive fields obfuscated.
pg_stat_recovery_prefetch #
The pg_stat_recovery_prefetch view will contain only one row. The columns wal_distance, block_distance and io_depth show current values, and the other columns show cumulative counters that can be reset with the pg_stat_reset_shared function.
Table 27.17. pg_stat_recovery_prefetch View
Column Type
Description
stats_reset timestamp with time zone
Time at which these statistics were last reset
prefetch bigint
Number of blocks prefetched because they were not in the buffer pool
hit bigint
Number of blocks not prefetched because they were already in the buffer pool
skip_init bigint
Number of blocks not prefetched because they would be zero-initialized
skip_new bigint
Number of blocks not prefetched because they didn't exist yet
skip_fpw bigint
Number of blocks not prefetched because a full page image was included in the WAL
skip_rep bigint
Number of blocks not prefetched because they were already recently prefetched
wal_distance int
How many bytes ahead the prefetcher is looking
block_distance int
How many blocks ahead the prefetcher is looking
io_depth int
How many prefetches have been initiated but are not yet known to have completed
pg_stat_subscription #
Table 27.18. pg_stat_subscription View
Column Type
Description
subid oid
OID of the subscription
subname name
Name of the subscription
worker_type text
Type of the subscription worker process. Possible types are apply, parallel apply, and table synchronization.
pid integer
Process ID of the subscription worker process
leader_pid integer
Process ID of the leader apply worker if this process is a parallel apply worker; NULL if this process is a leader apply worker or a table synchronization worker
relid oid
OID of the relation that the worker is synchronizing; NULL for the leader apply worker and parallel apply workers
received_lsn pg_lsn
Last write-ahead log location received, the initial value of this field being 0; NULL for parallel apply workers
last_msg_send_time timestamp with time zone
Send time of last message received from origin WAL sender; NULL for parallel apply workers
last_msg_receipt_time timestamp with time zone
Receipt time of last message received from origin WAL sender; NULL for parallel apply workers
latest_end_lsn pg_lsn
Last write-ahead log location reported to origin WAL sender; NULL for parallel apply workers
latest_end_time timestamp with time zone
Time of last write-ahead log location reported to origin WAL sender; NULL for parallel apply workers
pg_stat_subscription_stats #
The pg_stat_subscription_stats view will contain one row per subscription.
Table 27.19. pg_stat_subscription_stats View
Column Type
Description
subid oid
OID of the subscription
subname name
Name of the subscription
apply_error_count bigint
Number of times an error occurred while applying changes. Note that any conflict resulting in an apply error will be counted in both apply_error_count and the corresponding conflict count (e.g., confl_*).
sync_error_count bigint
Number of times an error occurred during the initial table synchronization
confl_insert_exists bigint
Number of times a row insertion violated a NOT DEFERRABLE unique constraint during the application of changes. See insert_exists for details about this conflict.
confl_update_origin_differs bigint
Number of times an update was applied to a row that had been previously modified by another source during the application of changes. See update_origin_differs for details about this conflict.
confl_update_exists bigint
Number of times that an updated row value violated a NOT DEFERRABLE unique constraint during the application of changes. See update_exists for details about this conflict.
confl_update_deleted bigint
Number of times the tuple to be updated was concurrently deleted by another source during the application of changes. See update_deleted for details about this conflict.
confl_update_missing bigint
Number of times the tuple to be updated was not found during the application of changes. See update_missing for details about this conflict.
confl_delete_origin_differs bigint
Number of times a delete operation was applied to row that had been previously modified by another source during the application of changes. See delete_origin_differs for details about this conflict.
confl_delete_missing bigint
Number of times the tuple to be deleted was not found during the application of changes. See delete_missing for details about this conflict.
confl_multiple_unique_conflicts bigint
Number of times a row insertion or an updated row values violated multiple NOT DEFERRABLE unique constraints during the application of changes. See multiple_unique_conflicts for details about this conflict.
stats_reset timestamp with time zone
Time at which these statistics were last reset
The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection.
Table 27.20. pg_stat_ssl View
Column Type
Description
pid integer
Process ID of a backend or WAL sender process
ssl boolean
True if SSL is used on this connection
version text
Version of SSL in use, or NULL if SSL is not in use on this connection
cipher text
Name of SSL cipher in use, or NULL if SSL is not in use on this connection
bits integer
Number of bits in the encryption algorithm used, or NULL if SSL is not used on this connection
client_dn text
Distinguished Name (DN) field from the client certificate used, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated if the DN field is longer than NAMEDATALEN (64 characters in a standard build).
client_serial numeric
Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. The combination of certificate serial number and certificate issuer uniquely identifies a certificate (unless the issuer erroneously reuses serial numbers).
issuer_dn text
DN of the issuer of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated like client_dn.
pg_stat_gssapi #
The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection.
Table 27.21. pg_stat_gssapi View
Column Type
Description
pid integer
Process ID of a backend
gss_authenticated boolean
True if GSSAPI authentication was used for this connection
principal text
Principal used to authenticate this connection, or NULL if GSSAPI was not used to authenticate this connection. This field is truncated if the principal is longer than NAMEDATALEN (64 characters in a standard build).
encrypted boolean
True if GSSAPI encryption is in use on this connection
credentials_delegated boolean
True if GSSAPI credentials were delegated on this connection.
pg_stat_archiver #
The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster.
Table 27.22. pg_stat_archiver View
Column Type
Description
archived_count bigint
Number of WAL files that have been successfully archived
last_archived_wal text
Name of the WAL file most recently successfully archived
last_archived_time timestamp with time zone
Time of the most recent successful archive operation
failed_count bigint
Number of failed attempts for archiving WAL files
last_failed_wal text
Name of the WAL file of the most recent failed archival operation
last_failed_time timestamp with time zone
Time of the most recent failed archival operation
stats_reset timestamp with time zone
Time at which these statistics were last reset
Normally, WAL files are archived in order, oldest to newest, but that is not guaranteed, and does not hold under special circumstances like when promoting a standby or after crash recovery. Therefore it is not safe to assume that all files older than last_archived_wal have also been successfully archived.
The pg_stat_io view will contain one row for each combination of backend type, target I/O object, and I/O context, showing cluster-wide I/O statistics. Combinations which do not make sense are omitted.
Currently, I/O on relations (e.g. tables, indexes) and WAL activity are tracked. However, relation I/O which bypasses shared buffers (e.g. when moving a table from one tablespace to another) is currently not tracked.
Table 27.23. pg_stat_io View
Column Type
Description
backend_type text
Type of backend (e.g. background worker, autovacuum worker). See pg_stat_activity for more information on backend_types. Some backend_types do not accumulate I/O operation statistics and will not be included in the view.
object text
Target object of an I/O operation. Possible values are:
relation: Permanent relations.
temp relation: Temporary relations.
wal: Write Ahead Logs.
context text
The context of an I/O operation. Possible values are:
normal: The default or standard context for a type of I/O operation. For example, by default, relation data is read into and written out from shared buffers. Thus, reads and writes of relation data to and from shared buffers are tracked in context normal.
init: I/O operations performed while creating the WAL segments are tracked in context init.
vacuum: I/O operations performed outside of shared buffers while vacuuming and analyzing permanent relations. Temporary table vacuums use the same local buffer pool as other temporary table I/O operations and are tracked in context normal.
bulkread: Certain large read I/O operations done outside of shared buffers, for example, a sequential scan of a large table.
bulkwrite: Certain large write I/O operations done outside of shared buffers, such as COPY.
reads bigint
Number of read operations.
read_bytes numeric
The total size of read operations in bytes.
read_time double precision
Time spent waiting for read operations in milliseconds (if track_io_timing is enabled and object is not wal, or if track_wal_io_timing is enabled and object is wal, otherwise zero)
writes bigint
Number of write operations.
write_bytes numeric
The total size of write operations in bytes.
write_time double precision
Time spent waiting for write operations in milliseconds (if track_io_timing is enabled and object is not wal, or if track_wal_io_timing is enabled and object is wal, otherwise zero)
writebacks bigint
Number of units of size BLCKSZ (typically 8kB) which the process requested the kernel write out to permanent storage.
writeback_time double precision
Time spent waiting for writeback operations in milliseconds (if track_io_timing is enabled, otherwise zero). This includes the time spent queueing write-out requests and, potentially, the time spent to write out the dirty data.
extends bigint
Number of relation extend operations.
extend_bytes numeric
The total size of relation extend operations in bytes.
extend_time double precision
Time spent waiting for extend operations in milliseconds. (if track_io_timing is enabled and object is not wal, or if track_wal_io_timing is enabled and object is wal, otherwise zero)
hits bigint
The number of times a desired block was found in a shared buffer.
evictions bigint
Number of times a block has been written out from a shared or local buffer in order to make it available for another use.
In context normal, this counts the number of times a block was evicted from a buffer and replaced with another block. In contexts bulkwrite, bulkread, and vacuum, this counts the number of times a block was evicted from shared buffers in order to add the shared buffer to a separate, size-limited ring buffer for use in a bulk I/O operation.
reuses bigint
The number of times an existing buffer in a size-limited ring buffer outside of shared buffers was reused as part of an I/O operation in the bulkread, bulkwrite, or vacuum contexts.
fsyncs bigint
Number of fsync calls. These are only tracked in context normal.
fsync_time double precision
Time spent waiting for fsync operations in milliseconds (if track_io_timing is enabled and object is not wal, or if track_wal_io_timing is enabled and object is wal, otherwise zero)
stats_reset timestamp with time zone
Time at which these statistics were last reset.
Some backend types never perform I/O operations on some I/O objects and/or in some I/O contexts. These rows are omitted from the view. For example, the checkpointer does not checkpoint temporary tables, so there will be no rows for backend_type checkpointer and object temp relation.
In addition, some I/O operations will never be performed either by certain backend types or on certain I/O objects and/or in certain I/O contexts. These cells will be NULL. For example, temporary tables are not fsynced, so fsyncs will be NULL for object temp relation. Also, the background writer does not perform reads, so reads will be NULL in rows for backend_type background writer.
For the object wal, fsyncs and fsync_time track the fsync activity of WAL files done in issue_xlog_fsync. writes and write_time track the write activity of WAL files done in XLogWrite. See Section 28.5 for more information.
pg_stat_io can be used to inform database tuning. For example:
A high evictions count can indicate that shared buffers should be increased.
Client backends rely on the checkpointer to ensure data is persisted to permanent storage. Large numbers of fsyncs by client backends could indicate a misconfiguration of shared buffers or of the checkpointer. More information on configuring the checkpointer can be found in Section 28.5.
Normally, client backends should be able to rely on auxiliary processes like the checkpointer and the background writer to write out dirty data as much as possible. Large numbers of writes by client backends could indicate a misconfiguration of shared buffers or of the checkpointer. More information on configuring the checkpointer can be found in Section 28.5.
Columns tracking I/O wait time will only be non-zero when track_io_timing is enabled. The user should be careful when referencing these columns in combination with their corresponding I/O operations in case track_io_timing was not enabled for the entire time since the last stats reset.
pg_stat_bgwriter #
The pg_stat_bgwriter view will always have a single row, containing data about the background writer of the cluster.
Table 27.24. pg_stat_bgwriter View
Column Type
Description
buffers_clean bigint
Number of buffers written by the background writer
maxwritten_clean bigint
Number of times the background writer stopped a cleaning scan because it had written too many buffers
buffers_alloc bigint
Number of buffers allocated
stats_reset timestamp with time zone
Time at which these statistics were last reset
pg_stat_checkpointer #
The pg_stat_checkpointer view will always have a single row, containing data about the checkpointer process of the cluster.
Table 27.25. pg_stat_checkpointer View
Column Type
Description
num_timed bigint
Number of scheduled checkpoints due to timeout
num_requested bigint
Number of requested checkpoints
num_done bigint
Number of checkpoints that have been performed
restartpoints_timed bigint
Number of scheduled restartpoints due to timeout or after a failed attempt to perform it
restartpoints_req bigint
Number of requested restartpoints
restartpoints_done bigint
Number of restartpoints that have been performed
write_time double precision
Total amount of time that has been spent in the portion of processing checkpoints and restartpoints where files are written to disk, in milliseconds
sync_time double precision
Total amount of time that has been spent in the portion of processing checkpoints and restartpoints where files are synchronized to disk, in milliseconds
buffers_written bigint
Number of shared buffers written during checkpoints and restartpoints
slru_written bigint
Number of SLRU buffers written during checkpoints and restartpoints
stats_reset timestamp with time zone
Time at which these statistics were last reset
Checkpoints may be skipped if the server has been idle since the last one. num_timed and num_requested count both completed and skipped checkpoints, while num_done tracks only the completed ones. Similarly, restartpoints may be skipped if the last replayed checkpoint record is already the last restartpoint. restartpoints_timed and restartpoints_req count both completed and skipped restartpoints, while restartpoints_done tracks only the completed ones.
The pg_stat_wal view will always have a single row, containing data about WAL activity of the cluster.
Table 27.26. pg_stat_wal View
Column Type
Description
wal_records bigint
Total number of WAL records generated
wal_fpi bigint
Total number of WAL full page images generated
wal_bytes numeric
Total amount of WAL generated in bytes
wal_buffers_full bigint
Number of times WAL data was written to disk because WAL buffers became full
stats_reset timestamp with time zone
Time at which these statistics were last reset
pg_stat_database #
The pg_stat_database view will contain one row for each database in the cluster, plus one for shared objects, showing database-wide statistics.
Table 27.27. pg_stat_database View
Column Type
Description
datid oid
OID of this database, or 0 for objects belonging to a shared relation
datname name
Name of this database, or NULL for shared objects.
numbackends integer
Number of backends currently connected to this database, or NULL for shared objects. This is the only column in this view that returns a value reflecting current state; all other columns return the accumulated values since the last reset.
xact_commit bigint
Number of transactions in this database that have been committed
xact_rollback bigint
Number of transactions in this database that have been rolled back
blks_read bigint
Number of disk blocks read in this database
blks_hit bigint
Number of times disk blocks were found already in the buffer cache, so that a read was not necessary (this only includes hits in the PostgreSQL buffer cache, not the operating system's file system cache)
tup_returned bigint
Number of live rows fetched by sequential scans and index entries returned by index scans in this database
tup_fetched bigint
Number of live rows fetched by index scans in this database
tup_inserted bigint
Number of rows inserted by queries in this database
tup_updated bigint
Number of rows updated by queries in this database
tup_deleted bigint
Number of rows deleted by queries in this database
conflicts bigint
Number of queries canceled due to conflicts with recovery in this database. (Conflicts occur only on standby servers; see pg_stat_database_conflicts for details.)
temp_files bigint
Number of temporary files created by queries in this database. All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the log_temp_files setting.
temp_bytes bigint
Total amount of data written to temporary files by queries in this database. All temporary files are counted, regardless of why the temporary file was created, and regardless of the log_temp_files setting.
deadlocks bigint
Number of deadlocks detected in this database
checksum_failures bigint
Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are disabled.
checksum_last_failure timestamp with time zone
Time at which the last data page checksum failure was detected in this database (or on a shared object), or NULL if data checksums are disabled.
blk_read_time double precision
Time spent reading data file blocks by backends in this database, in milliseconds (if track_io_timing is enabled, otherwise zero)
blk_write_time double precision
Time spent writing data file blocks by backends in this database, in milliseconds (if track_io_timing is enabled, otherwise zero)
session_time double precision
Time spent by database sessions in this database, in milliseconds (note that statistics are only updated when the state of a session changes, so if sessions have been idle for a long time, this idle time won't be included)
active_time double precision
Time spent executing SQL statements in this database, in milliseconds (this corresponds to the states active and fastpath function call in pg_stat_activity)
idle_in_transaction_time double precision
Time spent idling while in a transaction in this database, in milliseconds (this corresponds to the states idle in transaction and idle in transaction (aborted) in pg_stat_activity)
sessions bigint
Total number of sessions established to this database
sessions_abandoned bigint
Number of database sessions to this database that were terminated because connection to the client was lost
sessions_fatal bigint
Number of database sessions to this database that were terminated by fatal errors
sessions_killed bigint
Number of database sessions to this database that were terminated by operator intervention
parallel_workers_to_launch bigint
Number of parallel workers planned to be launched by queries on this database
parallel_workers_launched bigint
Number of parallel workers launched by queries on this database
stats_reset timestamp with time zone
Time at which these statistics were last reset
pg_stat_database_conflicts #
The pg_stat_database_conflicts view will contain one row per database, showing database-wide statistics about query cancels occurring due to conflicts with recovery on standby servers. This view will only contain information on standby servers, since conflicts do not occur on primary servers.
Table 27.28. pg_stat_database_conflicts View
Column Type
Description
datid oid
OID of a database
datname name
Name of this database
confl_tablespace bigint
Number of queries in this database that have been canceled due to dropped tablespaces
confl_lock bigint
Number of queries in this database that have been canceled due to lock timeouts
confl_snapshot bigint
Number of queries in this database that have been canceled due to old snapshots
confl_bufferpin bigint
Number of queries in this database that have been canceled due to pinned buffers
confl_deadlock bigint
Number of queries in this database that have been canceled due to deadlocks
confl_active_logicalslot bigint
Number of uses of logical slots in this database that have been canceled due to old snapshots or too low a wal_level on the primary
pg_stat_all_tables #
The pg_stat_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about accesses to that specific table. The pg_stat_user_tables and pg_stat_sys_tables views contain the same information, but filtered to only show user and system tables respectively.
Table 27.29. pg_stat_all_tables View
Column Type
Description
relid oid
OID of a table
schemaname name
Name of the schema that this table is in
relname name
Name of this table
seq_scan bigint
Number of sequential scans initiated on this table
last_seq_scan timestamp with time zone
The time of the last sequential scan on this table, based on the most recent transaction stop time
seq_tup_read bigint
Number of live rows fetched by sequential scans
idx_scan bigint
Number of index scans initiated on this table
last_idx_scan timestamp with time zone
The time of the last index scan on this table, based on the most recent transaction stop time
idx_tup_fetch bigint
Number of live rows fetched by index scans
n_tup_ins bigint
Total number of rows inserted
n_tup_upd bigint
Total number of rows updated. (This includes row updates counted in n_tup_hot_upd and n_tup_newpage_upd, and remaining non-HOT updates.)
n_tup_del bigint
Total number of rows deleted
n_tup_hot_upd bigint
Number of rows HOT updated. These are updates where no successor versions are required in indexes.
n_tup_newpage_upd bigint
Number of rows updated where the successor version goes onto a new heap page, leaving behind an original version with a t_ctid field that points to a different heap page. These are always non-HOT updates.
n_live_tup bigint
Estimated number of live rows
n_dead_tup bigint
Estimated number of dead rows
n_mod_since_analyze bigint
Estimated number of rows modified since this table was last analyzed
n_ins_since_vacuum bigint
Estimated number of rows inserted since this table was last vacuumed (not counting VACUUM FULL)
last_vacuum timestamp with time zone
Last time at which this table was manually vacuumed (not counting VACUUM FULL)
last_autovacuum timestamp with time zone
Last time at which this table was vacuumed by the autovacuum daemon
last_analyze timestamp with time zone
Last time at which this table was manually analyzed
last_autoanalyze timestamp with time zone
Last time at which this table was analyzed by the autovacuum daemon
vacuum_count bigint
Number of times this table has been manually vacuumed (not counting VACUUM FULL)
autovacuum_count bigint
Number of times this table has been vacuumed by the autovacuum daemon
analyze_count bigint
Number of times this table has been manually analyzed
autoanalyze_count bigint
Number of times this table has been analyzed by the autovacuum daemon
total_vacuum_time double precision
Total time this table has been manually vacuumed, in milliseconds (not counting VACUUM FULL). (This includes the time spent sleeping due to cost-based delays.)
total_autovacuum_time double precision
Total time this table has been vacuumed by the autovacuum daemon, in milliseconds. (This includes the time spent sleeping due to cost-based delays.)
total_analyze_time double precision
Total time this table has been manually analyzed, in milliseconds. (This includes the time spent sleeping due to cost-based delays.)
total_autoanalyze_time double precision
Total time this table has been analyzed by the autovacuum daemon, in milliseconds. (This includes the time spent sleeping due to cost-based delays.)
pg_stat_all_indexes #
The pg_stat_all_indexes view will contain one row for each index in the current database, showing statistics about accesses to that specific index. The pg_stat_user_indexes and pg_stat_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.
Table 27.30. pg_stat_all_indexes View
Column Type
Description
relid oid
OID of the table for this index
indexrelid oid
OID of this index
schemaname name
Name of the schema this index is in
relname name
Name of the table for this index
indexrelname name
Name of this index
idx_scan bigint
Number of index scans initiated on this index
last_idx_scan timestamp with time zone
The time of the last scan on this index, based on the most recent transaction stop time
idx_tup_read bigint
Number of index entries returned by scans on this index
idx_tup_fetch bigint
Number of live table rows fetched by simple index scans using this index
Indexes can be used by simple index scans, “bitmap” index scans, and the optimizer. In a bitmap scan the output of several indexes can be combined via AND or OR rules, so it is difficult to associate individual heap row fetches with specific indexes when a bitmap scan is used. Therefore, a bitmap scan increments the pg_stat_all_indexes.idx_tup_read count(s) for the index(es) it uses, and it increments the pg_stat_all_tables.idx_tup_fetch count for the table, but it does not affect pg_stat_all_indexes.idx_tup_fetch. The optimizer also accesses indexes to check for supplied constants whose values are outside the recorded range of the optimizer statistics because the optimizer statistics might be stale.
The idx_tup_read and idx_tup_fetch counts can be different even without any use of bitmap scans, because idx_tup_read counts index entries retrieved from the index while idx_tup_fetch counts live rows fetched from the table. The latter will be less if any dead or not-yet-committed rows are fetched using the index, or if any heap fetches are avoided by means of an index-only scan.
Index scans may sometimes perform multiple index searches per execution. Each index search increments pg_stat_all_indexes.idx_scan, so it's possible for the count of index scans to significantly exceed the total number of index scan executor node executions.
This can happen with queries that use certain SQL constructs to search for rows matching any value out of a list or array of multiple scalar values (see Section 9.25). It can also happen to queries with a column_name = value1 OR column_name = value2 ...
EXPLAIN ANALYZE outputs the total number of index searches performed by each index scan node. See Section 14.1.2 for an example demonstrating how this works.
pg_statio_all_tables #
The pg_statio_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about I/O on that specific table. The pg_statio_user_tables and pg_statio_sys_tables views contain the same information, but filtered to only show user and system tables respectively.
Table 27.31. pg_statio_all_tables View
Column Type
Description
relid oid
OID of a table
schemaname name
Name of the schema that this table is in
relname name
Name of this table
heap_blks_read bigint
Number of disk blocks read from this table
heap_blks_hit bigint
Number of buffer hits in this table
idx_blks_read bigint
Number of disk blocks read from all indexes on this table
idx_blks_hit bigint
Number of buffer hits in all indexes on this table
toast_blks_read bigint
Number of disk blocks read from this table's TOAST table (if any)
toast_blks_hit bigint
Number of buffer hits in this table's TOAST table (if any)
tidx_blks_read bigint
Number of disk blocks read from this table's TOAST table indexes (if any)
tidx_blks_hit bigint
Number of buffer hits in this table's TOAST table indexes (if any)
pg_statio_all_indexes #
The pg_statio_all_indexes view will contain one row for each index in the current database, showing statistics about I/O on that specific index. The pg_statio_user_indexes and pg_statio_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.
Table 27.32. pg_statio_all_indexes View
Column Type
Description
relid oid
OID of the table for this index
indexrelid oid
OID of this index
schemaname name
Name of the schema this index is in
relname name
Name of the table for this index
indexrelname name
Name of this index
idx_blks_read bigint
Number of disk blocks read from this index
idx_blks_hit bigint
Number of buffer hits in this index
pg_statio_all_sequences #
The pg_statio_all_sequences view will contain one row for each sequence in the current database, showing statistics about I/O on that specific sequence.
Table 27.33. pg_statio_all_sequences View
Column Type
Description
relid oid
OID of a sequence
schemaname name
Name of the schema this sequence is in
relname name
Name of this sequence
blks_read bigint
Number of disk blocks read from this sequence
blks_hit bigint
Number of buffer hits in this sequence
pg_stat_user_functions #
The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. The track_functions parameter controls exactly which functions are tracked.
Table 27.34. pg_stat_user_functions View
Column Type
Description
funcid oid
OID of a function
schemaname name
Name of the schema this function is in
funcname name
Name of this function
calls bigint
Number of times this function has been called
total_time double precision
Total time spent in this function and all other functions called by it, in milliseconds
self_time double precision
Total time spent in this function itself, not including other functions called by it, in milliseconds
PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages.
For each SLRU cache that's part of the core server, there is a configuration parameter that controls its size, with the suffix _buffers appended.
Table 27.35. pg_stat_slru View
Column Type
Description
name text
Name of the SLRU
blks_zeroed bigint
Number of blocks zeroed during initializations
blks_hit bigint
Number of times disk blocks were found already in the SLRU, so that a read was not necessary (this only includes hits in the SLRU, not the operating system's file system cache)
blks_read bigint
Number of disk blocks read for this SLRU
blks_written bigint
Number of disk blocks written for this SLRU
blks_exists bigint
Number of blocks checked for existence for this SLRU
flushes bigint
Number of flushes of dirty data for this SLRU
truncates bigint
Number of truncates for this SLRU
stats_reset timestamp with time zone
Time at which these statistics were last reset
Other ways of looking at the statistics can be set up by writing queries that use the same underlying statistics access functions used by the standard views shown above. For details such as the functions' names, consult the definitions of the standard views. (For example, in psql you could issue \d+ pg_stat_activity.) The access functions for per-database statistics take a database OID as an argument to identify which database to report on. The per-table and per-index functions take a table or index OID. The functions for per-function statistics take a function OID. Note that only tables, indexes, and functions in the current database can be seen with these functions.
Additional functions related to the cumulative statistics system are listed in Table 27.36.
Table 27.36. Additional Statistics Functions
Function
Description
pg_backend_pid () → integer
Returns the process ID of the server process attached to the current session.
pg_stat_get_backend_io ( integer ) → setof record
Returns I/O statistics about the backend with the specified process ID. The output fields are exactly the same as the ones in the pg_stat_io view.
The function does not return I/O statistics for the checkpointer, the background writer, the startup process and the autovacuum launcher as they are already visible in the pg_stat_io view and there is only one of each.
pg_stat_get_activity ( integer ) → setof record
Returns a record of information about the backend with the specified process ID, or one record for each active backend in the system if NULL is specified. The fields returned are a subset of those in the pg_stat_activity view.
pg_stat_get_backend_wal ( integer ) → record
Returns WAL statistics about the backend with the specified process ID. The output fields are exactly the same as the ones in the pg_stat_wal view.
The function does not return WAL statistics for the checkpointer, the background writer, the startup process and the autovacuum launcher.
pg_stat_get_snapshot_timestamp () → timestamp with time zone
Returns the timestamp of the current statistics snapshot, or NULL if no statistics snapshot has been taken. A snapshot is taken the first time cumulative statistics are accessed in a transaction if stats_fetch_consistency is set to snapshot
pg_stat_get_xact_blocks_fetched ( oid ) → bigint
Returns the number of block read requests for table or index, in the current transaction. This number minus pg_stat_get_xact_blocks_hit gives the number of kernel read() calls; the number of actual physical reads is usually lower due to kernel-level buffering.
pg_stat_get_xact_blocks_hit ( oid ) → bigint
Returns the number of block read requests for table or index, in the current transaction, found in cache (not triggering kernel read() calls).
pg_stat_clear_snapshot () → void
Discards the current statistics snapshot or cached information.
Resets all statistics counters for the current database to zero.
This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_stat_reset_shared ( [ target text DEFAULT NULL ] ) → void
Resets some cluster-wide statistics counters to zero, depending on the argument. target can be:
archiver: Reset all the counters shown in the pg_stat_archiver view.
bgwriter: Reset all the counters shown in the pg_stat_bgwriter view.
checkpointer: Reset all the counters shown in the pg_stat_checkpointer view.
io: Reset all the counters shown in the pg_stat_io view.
recovery_prefetch: Reset all the counters shown in the pg_stat_recovery_prefetch view.
slru: Reset all the counters shown in the pg_stat_slru view.
wal: Reset all the counters shown in the pg_stat_wal view.
NULL or not specified: All the counters from the views listed above are reset.
This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_stat_reset_single_table_counters ( oid ) → void
Resets statistics for a single table or index in the current database or shared across all databases in the cluster to zero.
This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_stat_reset_backend_stats ( integer ) → void
Resets statistics for a single backend with the specified process ID to zero.
This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_stat_reset_single_function_counters ( oid ) → void
Resets statistics for a single function in the current database to zero.
This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_stat_reset_slru ( [ target text DEFAULT NULL ] ) → void
Resets statistics to zero for a single SLRU cache, or for all SLRUs in the cluster. If target is NULL or is not specified, all the counters shown in the pg_stat_slru view for all SLRU caches are reset. The argument can be one of commit_timestamp, multixact_member, multixact_offset, notify, serializable, subtransaction, or transaction to reset the counters for only that entry. If the argument is other (or indeed, any unrecognized name), then the counters for all other SLRU caches, such as extension-defined caches, are reset.
This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_stat_reset_replication_slot ( text ) → void
Resets statistics of the replication slot defined by the argument. If the argument is NULL, resets statistics for all the replication slots.
This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
pg_stat_reset_subscription_stats ( oid ) → void
Resets statistics for a single subscription shown in the pg_stat_subscription_stats view to zero. If the argument is NULL, reset statistics for all subscriptions.
This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function.
Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. A database-wide ANALYZE is recommended after the statistics have been reset.
pg_stat_get_activity, the underlying function of the pg_stat_activity view, returns a set of records containing all the available information about each backend process. Sometimes it may be more convenient to obtain just a subset of this information. In such cases, another set of per-backend statistics access functions can be used; these are shown in Table 27.37. These access functions use the session's backend ID number, which is a small integer (>= 0) that is distinct from the backend ID of any concurrent session, although a session's ID can be recycled as soon as it exits. The backend ID is used, among other things, to identify the session's temporary schema if it has one. The function pg_stat_get_backend_idset provides a convenient way to list all the active backends' ID numbers for invoking these functions. For example, to show the PIDs and current queries of all backends:
SELECT pg_stat_get_backend_pid(backendid) AS pid,
pg_stat_get_backend_activity(backendid) AS query
FROM pg_stat_get_backend_idset() AS backendid;
Table 27.37. Per-Backend Statistics Functions
Function
Description
pg_stat_get_backend_activity ( integer ) → text
Returns the text of this backend's most recent query.
pg_stat_get_backend_activity_start ( integer ) → timestamp with time zone
Returns the time when the backend's most recent query was started.
pg_stat_get_backend_client_addr ( integer ) → inet
Returns the IP address of the client connected to this backend.
pg_stat_get_backend_client_port ( integer ) → integer
Returns the TCP port number that the client is using for communication.
pg_stat_get_backend_dbid ( integer ) → oid
Returns the OID of the database this backend is connected to.
pg_stat_get_backend_idset () → setof integer
Returns the set of currently active backend ID numbers.
pg_stat_get_backend_pid ( integer ) → integer
Returns the process ID of this backend.
pg_stat_get_backend_start ( integer ) → timestamp with time zone
Returns the time when this process was started.
pg_stat_get_backend_subxact ( integer ) → record
Returns a record of information about the subtransactions of the backend with the specified ID. The fields returned are subxact_count, which is the number of subtransactions in the backend's subtransaction cache, and subxact_overflow, which indicates whether the backend's subtransaction cache is overflowed or not.
pg_stat_get_backend_userid ( integer ) → oid
Returns the OID of the user logged into this backend.
pg_stat_get_backend_wait_event ( integer ) → text
Returns the wait event name if this backend is currently waiting, otherwise NULL. See Table 27.5 through Table 27.13.
pg_stat_get_backend_wait_event_type ( integer ) → text
Returns the wait event type name if this backend is currently waiting, otherwise NULL. See Table 27.4 for details.
pg_stat_get_backend_xact_start ( integer ) → timestamp with time zone
Returns the time when the backend's current transaction was started.
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