Troubleshooting Long-Running or Blocked Queries in PostgreSQL

Introduction
Identifying Long-Running and Blocked Queries
Example of SQL Query
Solutions to Resolve Blocking Issues
Conclusion

Introduction

In PostgreSQL databases, long-running or blocked queries can significantly impact application performance and user experience. Identifying and resolving these issues promptly is crucial to maintaining database efficiency.

Identifying Long-Running and Blocked Queries

To identify long-running or blocked queries in PostgreSQL, you can use a SQL query that retrieves information about both blocking and blocked processes. This query checks the system catalog views pg_locks and pg_stat_activity to find processes that are blocking others:

     SELECT
         blocking_locks.pid as blocking_pid,
         blocking_activity.usename as blocking_user,
         blocking_activity.state as blocking_state,
         blocking_activity.client_addr as blocking_ip,
         blocking_activity.client_hostname as blocking_hostname,
         blocking_activity.xact_start as blocking_xact_start,
         EXTRACT(EPOCH FROM (now() - blocking_activity.xact_start)) as blocking_xact_secs,
         blocking_activity.query as blocking_sql,
         ' -> ' as is_blocking,
         blocked_locks.pid as blocked_pid,
         blocked_activity.usename as blocked_user,
         blocked_activity.state as blocked_state,
         blocked_activity.client_addr as blocked_ip,
         blocked_activity.client_hostname as blocked_hostname,
         blocked_activity.xact_start as blocked_xact_start,
         EXTRACT(EPOCH FROM (now() - blocked_activity.xact_start)) as blocked_xact_secs,
         EXTRACT(EPOCH FROM (now() - blocked_activity.query_start)) as blocked_query_secs,
         blocked_activity.query as blocked_sql
     FROM
         pg_catalog.pg_locks blocked_locks
         JOIN pg_catalog.pg_stat_activity blocked_activity ON blocked_activity.pid = blocked_locks.pid
         JOIN pg_catalog.pg_locks blocking_locks ON (
             blocking_locks.locktype = blocked_locks.locktype
             AND blocking_locks.database is not distinct from blocked_locks.database
             AND blocking_locks.relation is not distinct from blocked_locks.relation
             AND blocking_locks.page is not distinct from blocked_locks.page
             AND blocking_locks.tuple is not distinct from blocked_locks.tuple
             AND blocking_locks.virtualxid is not distinct from blocked_locks.virtualxid
             AND blocking_locks.transactionid is not distinct from blocked_locks.transactionid
             AND blocking_locks.classid is not distinct from blocked_locks.classid
             AND blocking_locks.objid is not distinct from blocked_locks.objid
             AND blocking_locks.objsubid is not distinct from blocked_locks.objsubid
             AND blocking_locks.pid != blocked_locks.pid
         )
         JOIN pg_catalog.pg_stat_activity blocking_activity ON blocking_activity.pid = blocking_locks.pid
     WHERE
         NOT blocked_locks.granted
     ORDER BY
         blocked_query_secs DESC;
 

This query will display information about both the blocking and blocked queries, including their process IDs, users, states, client addresses, transaction start times, query durations, and SQL statements.

Example of SQL Query

Let's consider an example where a query is blocked by another process:

Assume that a SELECT query is waiting to acquire a lock on a table while another UPDATE query holds the lock. Using the above SQL query, you can identify both the blocking and blocked processes along with their respective details.

Solutions to Resolve Blocking Issues

Resolving blocking issues in PostgreSQL involves various approaches:

• Kill or terminate the blocking process: In some cases, terminating the blocking process may be necessary, especially if it's holding a lock unnecessarily for a prolonged period. For example:
  
  If a process with PID 12345 is blocking others, you can terminate it using:

  
  SELECT pg_terminate_backend(12345);
                      

• Optimize queries and transactions: Improving query performance and transaction management can reduce the likelihood of conflicts and blocking. For instance, optimizing a complex query or breaking it into smaller transactions can help:
  
  

  
  BEGIN;
    -- Execute optimized queries here
  COMMIT;
                      

• Use appropriate isolation levels: Choosing the correct transaction isolation level can mitigate blocking issues by managing how transactions interact with each other. For example, using READ COMMITTED instead of READ UNCOMMITTED can prevent dirty reads and reduce contention:
  
  

  
  SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
                      

• Monitor and tune database performance: Regularly monitor database performance metrics and tune configurations to optimize resource utilization and prevent contention. For instance, adjusting PostgreSQL configuration parameters such as max_connections and shared_buffers can improve concurrency and reduce blocking:
  
  

  
  ALTER SYSTEM SET max_connections = 200;
  ALTER SYSTEM SET shared_buffers = '4GB';
   -- Apply changes
  SELECT pg_reload_conf();
                      

By implementing these solutions and understanding the root causes of blocking, you can enhance the overall performance and stability of your PostgreSQL database.

Conclusion

Troubleshooting long-running or blocked queries in PostgreSQL requires careful analysis of system activity and locking mechanisms. Using SQL queries to identify blocking processes and applying appropriate solutions can effectively resolve performance issues and improve database efficiency.

Related content

• Troubleshooting Long-Running or Blocked Queries in PostgreSQL
• How to Analyze PostgreSQL Performance with Logging and pgBadger
• How to Retrieve Settings in PostgreSQL
• Optimizing PostgreSQL Performance: Tuning Shared Buffers
• PostgreSQL Vacuuming Best Practices
• Analyzing and Vacuuming Tables in PostgreSQL
• Best Practices for Managing PostgreSQL Statistics
• Understanding and Improving Table and Index Data Density in PostgreSQL
• Tracking SQL Statements with pg_stat_statements in PostgreSQL
• Retrieving Index Information in PostgreSQL
• Understanding and Analyzing Index Usage in PostgreSQL
• Understanding PostgreSQL Cache Hit Ratio

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