Identifying Blocking PostgreSQL Queries: A Guide for Database Administrators

This article explores a PostgreSQL query designed to identify currently blocked database queries and the processes (identified by PIDs) responsible for blocking them. This information is crucial for database administrators troubleshooting performance bottlenecks and ensuring smooth database operation.

Sample Code from Command Line

1SELECT
2  pid,
3  usename,
4  pg_blocking_pids(pid) AS blocked_by_pids,
5  query AS blocked_query
6FROM
7  pg_stat_activity
8WHERE
9  cardinality(pg_blocking_pids(pid)) > 0;

Notes: Lists PostgreSQL queries blocked along with the pids of those holding the locks blocking them. Requires PostgreSQL >= 9.6. Tested on PostgreSQL 9.6+, 10.x 11.x, 12.x, 13.0.

Identifying Blocking PostgreSQL Queries: A Guide for Database Administrators

This article explores a PostgreSQL query designed to identify currently blocked database queries and the processes (identified by PIDs) responsible for blocking them. This information is crucial for database administrators troubleshooting performance bottlenecks and ensuring smooth database operation.

Understanding the Code:

The provided PostgreSQL code utilizes the following functions and features:

• pg_stat_activity: This built-in function offers a real-time view of currently active PostgreSQL backend processes.
• pid: This column within pg_stat_activity represents the process identifier (PID) of each database session.
• usename: This column reveals the username associated with the database session.
• pg_blocking_pids(pid): This function accepts a PID as input and returns a comma-separated list of PIDs that are currently blocking the specified process.
• cardinality(): This function determines the number of elements within a set. In this case, it counts the PIDs returned by pg_blocking_pids(pid).
• WHERE: This clause filters the results based on the specified condition.

Key Breakdown:

1. Data Retrieval: The query retrieves data from the pg_stat_activity system view.
2. Selection: It selects specific columns:
   • pid: The process identifier of the blocked session.
   • usename: The username of the blocked session's owner.
   • pg_blocking_pids(pid) AS blocked_by_pids: This expression utilizes the pg_blocking_pids function to identify the PIDs of processes blocking the current session. The result is aliased as blocked_by_pids for better readability.
   • query AS blocked_query: This retrieves the actual query currently being blocked. The result is aliased as blocked_query for clarity.
3. Filtering: The WHERE clause ensures only processes with active blockers are included. It achieves this by checking if the cardinality (number of elements) of the blocked_by_pids list is greater than zero.

Insights and Explanations:

This query provides valuable insights into potential database performance issues:

• Blocked queries indicate processes waiting for resources held by other processes. This can lead to slowdowns and bottlenecks.
• Identifying the blocking PIDs allows pinpointing the root cause of the blocking issue.
• Examining the blocked_query can reveal the specific operations causing the blockage.

Optimizing Database Performance:

By analyzing the results of this query, database administrators can:

• Terminate long-running queries if necessary (with caution to avoid data loss).
• Optimize inefficient queries to reduce resource consumption.
• Identify potential database schema deadlocks and address them.

Conclusion:

This PostgreSQL query equips database administrators with a powerful tool to diagnose blocking queries and optimize database performance. By understanding the code's functionality and interpreting the results effectively, database administrators can ensure a smooth and responsive database environment for their applications.

References:

PostgreSQL Documentation on pg_stat_activity: https://www.postgresql.org/docs/current/monitoring-stats.html

PostgreSQL Documentation on pg_blocking_pids: https://pgpedia.info/p/pg_blocking_pids.html

Source:

https://github.com/HariSekhon/SQL-scripts/blob/master/postgres_blocked_queries.sql