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PostgreSQL - Errors and Messages

When working with PostgreSQL, handling errors effectively is important for ensuring the stability and reliability of database operations. PostgreSQL provides a strong error-handling mechanism, allowing developers to diagnose and resolve issues efficiently.

In this article, we will explore PostgreSQL error-handling mechanisms, including the RAISE statement and ASSERT statement, to help us manage and debug our database operations. Additionally, we'll provide practical examples and outputs to illustrate how to handle errors effectively, ensuring our database processes are smooth and error-free.

In PostgreSQL, errors may arise due to various reasons, including syntax errors, invalid data operations, or constraint violations. These errors can disrupt normal database operations if not managed correctly. PostgreSQL offers a range of tools and statements for error handling, allowing developers to log, debug, and resolve issues with ease.

• Syntax errors in SQL statements
  
• Constraint violations like unique_violation or foreign_key_violation
  
• Invalid column references

1. RAISE Statement: For generating custom error messages.
   
2. ASSERT Statement: For debugging and validating conditions during development.

The RAISE statement in PostgreSQL is a powerful tool used to generate messages or errors during the execution of PL/pgSQL code blocks. The RAISE statement offers a flexible way to control how messages are handled within our database environment.

To raise an error message user can implement the RAISE statement as follows:

RAISE level format;

level: Following the RAISE statement is the level option that specifies the error severity. PostgreSQL provides the following levels:

• DEBUG: For detailed debugging information.
  
• LOG: For logging purposes, typically used for recording events.
  
• NOTICE: For messages that inform the user of something non-critical.
  
• INFO: For informational messages that are typically less severe than warnings.
  
• WARNING: For messages that indicate potential issues that do not halt execution.
  
• EXCEPTION: For critical errors that stop the current transaction. This is the default level if none is specified.

If users don’t specify the level, by default, the RAISE statement will use the EXCEPTION level that raises an error and stops the current transaction.

• format: The format is a string that specifies the message. The format uses percentage ( %) placeholders that will be substituted by the next arguments.
  
• expression: Values that replace placeholders in the format string.

The number of placeholders must match the number of arguments, otherwise, PostgreSQL will report the following error message:

[Err] ERROR:  too many parameters specified for RAISE

The following example illustrates the RAISE statement that reports different messages at the current time.

DO $$ 
BEGIN 
  RAISE INFO 'information message %', now() ;
  RAISE LOG 'log message %', now();
  RAISE DEBUG 'debug message %', now();
  RAISE WARNING 'warning message %', now();
  RAISE NOTICE 'notice message %', now();
END $$;

Output

Explanation

• INFO, WARNING, and NOTICE level messages are reported back to the client.
  
• DEBUG and LOG level messages are generally written to the server log and not sent to the client.
  
• This behavior can be controlled using the 'client_min_messages' and 'log_min_messages' configuration parameters, allowing us to fine-tune the verbosity of the messages sent to the client versus those logged by the server.

To raise errors, we use the 'EXCEPTION' level after the RAISE statement. Note that the RAISE statement uses the EXCEPTION level by default. Besides raising an error, we can add more detailed information by using the following clause with the RAISE statement:

USING option = expression

The options can be any one of the below:

• MESSAGE: set error message text
  
• HINT: provide the hint message so that the root cause of the error is easier to be discovered.
  
• DETAIL: give detailed information about the error.
  
• ERRCODE: identify the error code, which can be either by condition name or directly five-character SQLSTATE code.

DO $$ 
DECLARE
  email varchar(255) := 'raju@geeksforgeeks.org';
BEGIN 
  -- check email for duplicate
  -- ...
  -- report duplicate email
  RAISE EXCEPTION 'Duplicate email: %', email 
        USING HINT = 'Check the email again';
END $$;

Output

Explanation:

This block raises an exception with a detailed hint to assist in identifying the issue.

The following examples illustrate how to raise an SQLSTATE and its corresponding condition:

DO $$ 
BEGIN 
    --...
    RAISE SQLSTATE '2201B';
END $$;
DO $$ 
BEGIN 
    --...
    RAISE invalid_regular_expression;
END $$;

Output

ERROR:  custom error message
SQL state: 2201B

Explanation:

• SQLSTATE '2201B' corresponds to a specific error code in SQL standard. This is used to raise an exception with this particular error code.
• We can also include a custom message using the USING MESSAGE = 'Your message here' clause, though it’s not mandatory.

The ASSERT statement in PostgreSQL allows us to insert debugging checks into our PL/pgSQL blocks. This is particularly useful during development to ensure that certain conditions hold true at specific points in our code.

Syntax

ASSERT condition [, 'message'];

Key Terms

• condition: A boolean expression that we expect to be true.
  
• message: (Optional) A custom error message displayed if the condition is false.

If any assertion fails, PostgreSQL will raise an error with the specified message, aiding in identifying logical issues during development.

DO $$ 
BEGIN 
  ASSERT 1 + 1 = 2, 'Math is broken!';
  ASSERT (SELECT COUNT(*) FROM users) > 0, 'No users found in the database';
END $$;

PostgreSQL provides detailed error messages in the client console or server logs. Developers can use the RAISE statement in PL/pgSQL or query system views like pg_stat_activity for error tracking.

1. Syntax errors in SQL queries.
   
2. Constraint violations (e.g., unique_violation, foreign_key_violation).
   
3. Invalid column or table references.

1. BEGIN...EXCEPTION...END blocks in PL/pgSQL for error handling.
   
2. Logging errors using configuration parameters like log_statement.
   
3. Monitoring logs for troubleshooting.

In conclusion, effective error handling is a cornerstone of powerful database management. PostgreSQL's error-handling mechanisms, such as the RAISE statement and ASSERT statement, offer powerful tools for managing errors and debugging code. Whether we're raising custom errors, providing detailed hints, or validating conditions during development, these tools empower developers to handle errors efficiently.

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