Part 198: Production Query Triage: Slow Queries, Locks & Kill Commands

What to do when the database CPU hits 100% and queries are hanging.

1. Finding Currently Running Long Queries

-- PostgreSQL: Find queries running longer than 30 seconds
SELECT 
    pid,
    now() - query_start AS duration,
    usename,
    state,
    wait_event_type,
    wait_event,
    query
FROM pg_stat_activity
WHERE state != 'idle' 
  AND (now() - query_start) > interval '30 seconds'
ORDER BY duration DESC;

2. Finding Blocking and Blocked Queries

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;

3. Safely Cancelling and Killing Rogue Queries

-- 1. Polite cancel (Interrupts query, leaves connection open):
SELECT pg_cancel_backend(12345);

-- 2. Force terminate (Terminates connection and rolls back transaction):
SELECT pg_terminate_backend(12345);

Key Takeaways

• Always try pg_cancel_backend() before resorting to hard pg_terminate_backend().

In Part 199, we explore 50 Staff & Principal Level SQL Interview Questions & Scenarios!