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.
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:
pg_locks
pg_stat_activity
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;
blocking
blocked queries
process IDs, users, states, client addresses, transaction start times, query durations
SQL statements
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.
SELECT
UPDATE
12345
SELECT pg_terminate_backend(12345);
BEGIN; -- Execute optimized queries here COMMIT;
READ COMMITTED
READ UNCOMMITTED
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
max_connections
shared_buffers
ALTER SYSTEM SET max_connections = 200; ALTER SYSTEM SET shared_buffers = '4GB'; -- Apply changes SELECT pg_reload_conf();
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.