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Query syntax in PostgreSQL | Spanner

This page defines the syntax of the SQL statement supported for PostgreSQL-dialect databases.

• Square brackets [ ] indicate optional clauses.
• Curly braces { } enclose a set of options.
• The vertical bar | indicates a logical OR.
• A comma followed by an ellipsis indicates that the preceding item can repeat in a comma-separated list.
  • item [, ...] indicates one or more items, and
  • [item, ...] indicates zero or more items.
• Purple-colored text, such as item, marks Spanner extensions to open source PostgreSQL.
• Parentheses ( ) indicate literal parentheses.
• A comma , indicates the literal comma.
• Angle brackets <> indicate literal angle brackets.
• Uppercase words, such as INSERT, are keywords.

Use the CALL statement to invoke a stored system procedure.

CALL procedure_name (procedure_argument[, …])

CALL executes a stored system procedure procedure_name. You can't create your own stored system procedure. You can only use system procedures. For more information, see stored system procedures.

The CALL statement uses the following parameters:

procedure_name

The name of the stored system procedure.

procedure_argument

An argument expression for the stored system procedure call.

Use the SELECT statement to retrieve data from a database.

[ /*@ hint_expression [, ...] */ ] [ WITH cte[, ...] ] [, ...] ] select

where cte is:

     cte_name AS ( select )

and select is:

    SELECT select-list
        [ FROM from_item [, ...] ]
        [ WHERE condition ]
        [ GROUP BY grouping_element [, ...] ]
        [ HAVING condition ]
        [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] select ]
        [ ORDER BY expression [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] ]
        [ LIMIT count ]
        [ OFFSET start ]
        [ FOR UPDATE ]

and select-list is:

    [ { ALL | DISTINCT } ] { * | expression [ [ AS ] output_name ] [, ...] }

and from_item is one of:

    table_name [ /*@ table_hint_expression [, ...] */ ]
         [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
    ( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
    from_item join_type [ /*@ join_hint_expression [, ...] */ ]
        from_item [ ON join_condition | USING ( join_column [, ...] ) ]
    from_item unnest_operator

and join_type is one of:

    [ INNER ] JOIN
    LEFT [ OUTER ] JOIN
    RIGHT [ OUTER ] JOIN
    FULL [ OUTER ] JOIN
    CROSS JOIN

and grouping_element is one of:

    ( )
    expression
    ( expression [, ...] )

and hint_expression is one of:

    statement_hint_key = statement_hint_value
    table_hint_key = table_hint_value
    join_hint_key = join_hint_value

and table_hint_expression is:

    table_hint_key = table_hint_value

and join_hint_expression is:

    join_hint_key = join_hint_value

and statement_hint_key is:

    USE_ADDITIONAL_PARALLELISM | LOCK_SCANNED_RANGES | SCAN_METHOD |
    EXECUTION_METHOD | ALLOW_TIMESTAMP_PREDICATE_PUSHDOWN

and table_hint_key is:

    FORCE_INDEX | GROUPBY_SCAN_OPTIMIZATION | SCAN_METHOD

and join_hint_key is:

    FORCE_JOIN_ORDER | JOIN_METHOD | HASH_JOIN_BUILD_SIDE | BATCH_MODE

cte_name AS ( select )

A common table expression (CTE) includes a CTE name and SELECT statement.

• A CTE cannot reference itself.
• A CTE can be referenced by the query expression that contains the WITH clause, but rules apply. Those rules are described later in this topic.

In this example, a WITH clause defines two CTEs that are referenced in the related set operation, where one CTE is referenced by each of the set operation's input query expressions:

WITH subQ1 AS (SELECT SchoolID FROM Roster),
     subQ2 AS (SELECT OpponentID FROM PlayerStats)
SELECT * FROM subQ1
UNION ALL
SELECT * FROM subQ2

WITH is not supported in a subquery. This returns an error:

SELECT account
FROM (
  WITH result AS (SELECT * FROM NPCs)
  SELECT *
  FROM result)

The WITH clause is not supported in DML statements.

Temporary tables defined by the WITH clause are stored in memory. Spanner dynamically allocates memory for all temporary tables created by a query. If the available resources are not sufficient then the query will fail.

Common table expressions (CTEs) can be referenced inside the query expression that contains the WITH clause.

Here are some general rules and constraints to consider when working with CTEs:

• Each CTE in the same WITH clause must have a unique name.
• A CTE defined in a WITH clause is only visible to other CTEs in the same WITH clause that were defined after it.
• A local CTE overrides an outer CTE or table with the same name.
• A CTE on a subquery may not reference correlated columns from the outer query.

References between CTEs in the WITH clause can be backward references, but not forward references.

The following is what happens when you have two CTEs that reference themselves or each other in a WITH clause. Assume that A is the first CTE and B is the second CTE in the clause:

• A references A = Invalid
• A references B = Invalid
• B references A = Valid
• A references B references A = Invalid (cycles are not allowed)

This produces an error. A cannot reference itself because self-references are not supported:

WITH
  A AS (SELECT 1 AS n UNION ALL (SELECT n + 1 FROM A WHERE n < 3))
SELECT * FROM A

-- Error

This produces an error. A cannot reference B because references between CTEs can go backwards but not forwards:

WITH
  A AS (SELECT * FROM B),
  B AS (SELECT 1 AS n)
SELECT * FROM B

-- Error

B can reference A because references between CTEs can go backwards:

WITH
  A AS (SELECT 1 AS n),
  B AS (SELECT * FROM A)
SELECT * FROM B

+---+
| n |
+---+
| 1 |
+---+

This produces an error. A and B reference each other, which creates a cycle:

WITH
  A AS (SELECT * FROM B),
  B AS (SELECT * FROM A)
SELECT * FROM B

-- Error

SELECT
...
FOR UPDATE;

When you use the SELECT query to scan a table, add a FOR UPDATE clause to enable exclusive locks at the row-and-column granularity level, otherwise known as cell-level. The lock remains in place for the lifetime of the read-write transaction. During this time, the FOR UPDATE clause prevents other transactions from modifying the locked cells until the current transaction completes.

To be consistent with other PostgreSQL-dialect databases, you can't use the FOR UPDATE clause with set operators UNION, INTERSECT and EXCEPT, combined with GROUP BY, HAVING and DISTINCT clauses, and aggregate functions like array_agg().

Example:

SELECT marketingbudget
FROM albums
WHERE singerid = 1 and albumid = 1
FOR UPDATE;

UPDATE albums
SET marketingbudget = 100000
WHERE singerid = 1 and albumid = 1;

You can't use the FOR UPDATE clause in the following ways:

• In combination with the LOCK_SCANNED_RANGES hint
• In read-only transactions
• Within DDL statements

For more information, see Use SELECT FOR UPDATE.

Spanner has extensions for statement hints, table hints, and join hints.

[ /*@ statement_hint_key = statement_hint_value [, ...] */ ]

where statement_hint_key is:

    USE_ADDITIONAL_PARALLELISM | LOCK_SCANNED_RANGES | SCAN_METHOD |
    EXECUTION_METHOD | ALLOW_TIMESTAMP_PREDICATE_PUSHDOWN

Spanner supports the following statement hints as extensions to open source PostgreSQL.

Because this can reduce resources available to other operations, you may want to avoid this hint if you run latency-sensitive operations on the same instance.

Without the hint, it's possible that multiple simultaneous transactions will acquire shared locks, and then try to upgrade to exclusive locks. This will cause a deadlock, because each transaction's shared lock is preventing the other transaction(s) from upgrading to exclusive. Spanner aborts all but one of the transactions.

When requesting an exclusive lock using this hint, one transaction acquires the lock and proceeds to execute, while other transactions wait their turn for the lock. Throughput is still limited because the conflicting transactions can only be performed one at a time, but in this case Spanner is always making progress on one transaction, saving time that would otherwise be spent aborting and retrying transactions.

This hint is supported on all statement types, both query and DML.

Spanner always enforces serializability. Lock mode hints can affect which transactions wait or abort in contended workloads, but don't change the isolation level.

Because this is just a hint, it shouldn't be considered equivalent to a mutex. In other words, you shouldn't use Spanner exclusive locks as a mutual exclusion mechanism for the execution of code outside of Spanner. For more information, see Locking.

You can't use both the FOR UPDATE clause and the LOCK_SCANNED_RANGES hint in the same query. An error is returned. For more information, see Use SELECT FOR UPDATE.

The default Spanner scan method is AUTO (automatic). The AUTO setting specifies that depending on the heuristics of the query, batch or row-oriented query processing might be used to improve query performance. If you want to change the default scanning method, you can use a statement hint to enforce the BATCH-oriented or ROW-oriented processing method. You can't manually set the scan method to AUTO. However, if you remove the statement hint, then Spanner uses the AUTO scan method. For more information, see Optimize scans.

The default Spanner query execution method is DEFAULT. The DEFAULT setting specifies that batch-oriented execution might be used to improve query performance, depending on the heuristics of the query. If you want to change the default execution method, you can use a statement hint to enforce the BATCH-oriented or ROW-oriented execution method. You can't manually set the query execution method to DEFAULT. However, if you remove the statement hint, then Spanner uses the DEFAULT execution method. For more information, see Optimize query execution.

[ /*@ table_hint_key = table_hint_value [, ...] */ ]

where table_hint_key is:

    FORCE_INDEX | GROUPBY_SCAN_OPTIMIZATION | SCAN_METHOD

Spanner supports the following table hints as extensions to open source PostgreSQL.

• If set to the name of an index, use that index instead of the base table. If the index cannot provide all needed columns, perform a back join with the base table.
• If set to the string _BASE_TABLE, use the base table for the index strategy instead of an index. Note that this is the only valid value when FORCE_INDEX is used in a statement hint expression.

Note: FORCE_INDEX is actually a directive, not a hint, which means an error is raised if the index does not exist.

The group by scan optimization can make queries faster if they use GROUP BY. It can be applied if the grouping keys can form a prefix of the underlying table or index key, and if the query requires only the first row from each group.

The optimization is applied if the optimizer estimates that it will make the query more efficient. The hint overrides that decision. If the hint is set to FALSE, the optimization is not considered. If the hint is set to TRUE, the optimization will be applied as long as it is legal to do so.

By default, Spanner sets the scan method as AUTO (automatic) which means depending on the heuristics of the query, batch-oriented query processing might be used to improve query performance. If you want to change the default scanning method from AUTO, you can use the hint to enforce a ROW or BATCH oriented processing method. For more information see Optimize scans.

[ /*@ join_hint_key = join_hint_value [, ...] */ ]

where join_hint_key is:

    FORCE_JOIN_ORDER | JOIN_METHOD | HASH_JOIN_BUILD_SIDE | BATCH_MODE

Spanner supports the following join hints as extensions to open source PostgreSQL.

function_name() [ /*@ function_hint_key = function_hint_value [, ...] */ ]

where function_hint_key is:

    DISABLE_INLINE

Spanner supports the following function hints as extensions to open source PostgreSQL.

If set to true, the function is computed once instead of each time another part of a query references it.

DISABLE_INLINE works with top-level functions.

You can't use DISABLE_INLINE with a few functions, including those that don't produce a scalar value and casting. Although you can't use DISABLE_INLINE with a casting function, you can use it with the first expression inside the function.

Examples

In the following example, inline expressions are enabled by default for x. x is computed twice, once by each reference:

SELECT
  SUBSTRING(x, 2, 5) AS w,
  SUBSTRING(x, 3, 7) AS y
FROM (SELECT SHA512(z) AS x FROM t) AS subquery

In the following example, inline expressions are disabled for x. x is computed once, and the result is used by each reference:

SELECT
  SUBSTRING(x, 2, 5) AS w,
  SUBSTRING(x, 3, 7) AS y
FROM (SELECT SHA512(z) /*@ DISABLE_INLINE = TRUE */ AS x FROM t) AS subquery

Join methods are specific implementations of the various logical join types. Some join methods are available only for certain join types. The choice of which join method to use depends on the specifics of your query and of the data being queried. The best way to figure out if a particular join method helps with the performance of your query is to try the method and view the resulting query execution plan. See Query Execution Operators for more details.

unnest_operator:
  {
    UNNEST( array_expression )
    | UNNEST( array_path )

  }
  [ table_hint_expr ]
  [ as_alias ]

as_alias:
  [AS] alias

The UNNEST operator takes an array and returns a table, with one row for each element in the array. For input arrays of most element types, the output of UNNEST generally has one column.

Input values:

• array_expression: an expression that produces an array.
• table_name: The name of a table.

• array_path: The path to an ARRAY type.

  Example:

  SELECT * FROM UNNEST (ARRAY[10,20,30]) as numbers;
  
  /*---------*
   | numbers |
   +---------+
   | 10      |
   | 20      |
   | 30      |
   *---------*/
  

• alias: An alias for a value table. An input array that produces a single column can have an optional alias, which you can use to refer to the column elsewhere in the query.

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