How to detect query which holds the lock in Postgres?

Question

I want to track mutual locks in postgres constantly.

I came across Locks Monitoring article and tried to run the following query:

SELECT bl.pid     AS blocked_pid,
     a.usename  AS blocked_user,
     kl.pid     AS blocking_pid,
     ka.usename AS blocking_user,
     a.query    AS blocked_statement
FROM  pg_catalog.pg_locks         bl
 JOIN pg_catalog.pg_stat_activity a  ON a.pid = bl.pid
 JOIN pg_catalog.pg_locks         kl ON kl.transactionid = bl.transactionid AND kl.pid != bl.pid
 JOIN pg_catalog.pg_stat_activity ka ON ka.pid = kl.pid
WHERE NOT bl.granted;

Unfortunately, it never returns non-empty result set. If I simplify given query to the following form:

SELECT bl.pid     AS blocked_pid,
     a.usename  AS blocked_user,
     a.query    AS blocked_statement
FROM  pg_catalog.pg_locks         bl
 JOIN pg_catalog.pg_stat_activity a  ON a.pid = bl.pid
WHERE NOT bl.granted;

then it returns queries which are waiting to acquire a lock. But I cannot manage to change it so that it can return both blocked and blocker queries.

Any ideas?

Answer 1

Since 9.6 this is a lot easier as it introduced the function pg_blocking_pids() to find the sessions that are blocking another session.

So you can use something like this:

select pid, 
       usename, 
       pg_blocking_pids(pid) as blocked_by, 
       query as blocked_query
from pg_stat_activity
where cardinality(pg_blocking_pids(pid)) > 0;