Keeping Your PostgreSQL Database in Check: Methods for Terminating Slow Queries


Finding the culprit:

  1. Identify the Process ID (PID) of the query you want to stop. Use the following query in psql to see active queries and their PIDs:
SELECT * FROM pg_stat_activity WHERE state = 'active';

This will return a list of running queries, including their PIDs.

Stopping the query:

There are two main functions for terminating queries, each with a different approach:

  1. pg_cancel_backend(PID): This function sends a polite request to the backend process (identified by PID) to cancel the current query. The query might take a moment to finish up any critical operations before stopping. This is the preferred method as it allows for a graceful termination.

  2. pg_terminate_backend(PID): This function takes a more forceful approach. It immediately terminates the backend process identified by PID. This will end the query and also close the database session associated with that PID. Use this option with caution as it can lead to data inconsistency if the query was modifying data when terminated.

Example (using pg_cancel_backend):

SELECT pg_cancel_backend(1234);  -- Replace 1234 with the actual PID

Additional considerations:

  • You can terminate all active queries (except your own) using a query like this:
SELECT pg_cancel_backend(pid) FROM pg_stat_activity WHERE state = 'active' AND pid <> pg_backend_pid();

Stopping a specific query using pg_cancel_backend:

-- Find the PID of the query to stop (replace 1234 with the actual PID)
SELECT * FROM pg_stat_activity WHERE state = 'active';

-- Stop the query using the PID
SELECT pg_cancel_backend(1234);

This code first retrieves information about active queries using pg_stat_activity. Then, it identifies the desired query by its PID (process ID) and uses pg_cancel_backend to send a termination request to that specific process.

Terminating all active queries except your own (use with caution):

SELECT pg_cancel_backend(pid) FROM pg_stat_activity WHERE state = 'active' AND pid <> pg_backend_pid();

This code iterates through all active queries (state = 'active') using pg_stat_activity. It then checks if the process ID (PID) is different from the current session's PID (pg_backend_pid()). If it's a different process, pg_cancel_backend is used to send a termination request to that specific query. This approach should be used cautiously as it can disrupt other users.

Forcefully terminating a query using pg_terminate_backend (use with extreme caution):

-- Find the PID of the query to terminate (replace 1234 with the actual PID)
SELECT * FROM pg_stat_activity WHERE state = 'active';

-- Forcefully terminate the query using the PID
SELECT pg_terminate_backend(1234);

This code is similar to the first example, but it uses pg_terminate_backend instead. This function immediately terminates the backend process identified by the PID, ending the query and closing the associated database session. Use this option with extreme caution as it can lead to data inconsistency.

  1. Setting Statement Timeouts:

PostgreSQL allows setting a statement timeout at the database session or server level. This automatically terminates any query exceeding the specified time limit. This is a proactive approach to prevent long-running queries from causing issues in the first place.

Here's how to set a statement timeout for the current session:

SET statement_timeout = 10000;  -- Timeout in milliseconds (10 seconds in this example)

You can set a server-wide default timeout by editing the postgresql.conf file and restarting the server. Refer to the PostgreSQL documentation https://postgresqlco.nf/doc/en/param/statement_timeout/ for details.

  1. Client-side Termination (if applicable):

If your application interacts with PostgreSQL through a client program or library, it might offer functionalities to terminate queries on the client-side. These functionalities might be specific to the client you're using. Consult the documentation for your client program/library to see if it provides such options.

  1. Optimize Queries:

The most effective way to avoid needing to stop queries might be to optimize them in the first place. Analyzing slow queries and optimizing them for better performance can significantly reduce the need for forceful termination. Tools like EXPLAIN in PostgreSQL can help identify bottlenecks and optimize queries.


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