PostgreSQL - Check blocking SQL statements

pg_locks view

Looking at pg_locks shows you what locks are granted and what processes are waiting for locks to be acquired. A good query to start looking for lock problems:

  select relation::regclass, * from pg_locks where not granted;

pg_stat_activity view

• Figuring out what the processes holding or waiting for locks is easier if you cross-reference against the information in pg_stat_activity

Сombination of blocked and blocking activity

The following query may be helpful to see what processes are blocking SQL statements (these only find row-level locks, not object-level locks).

  SELECT blocked_locks.pid     AS blocked_pid,
         blocked_activity.usename  AS blocked_user,
         blocking_locks.pid     AS blocking_pid,
         blocking_activity.usename AS blocking_user,
         blocked_activity.query    AS blocked_statement,
         blocking_activity.query   AS current_statement_in_blocking_process
   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;

Here's an alternate view of that same data that includes an idea how old the state is

SELECT a.datname,
       a.application_name,
       l.relation::regclass,
       l.transactionid,
       l.mode,
       l.locktype,
       l.GRANTED,
       a.usename,
       a.query,
       a.query_start,
       age(now(), a.query_start) AS "age",
       a.pid
FROM pg_stat_activity a
JOIN pg_locks l ON l.pid = a.pid
ORDER BY a.query_start;

Copied from: https://wiki.postgresql.org/wiki/Lock_Monitoring