Check the blocking sessions or queries in the PostgreSQL database

Check the user or process id which causes blocking:

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;

Check to block with the following query by using pg_blocking_pid function.

Pg_blocking_pid function is used to identify the blocking process id which causes another session to wait. Function returns array of PIDS which can be used in the following query with ANY clause.

SELECT
    activity.pid,
    activity.usename,
    activity.query,
    blocking.pid AS blocking_id,
    blocking.query AS blocking_query
FROM pg_stat_activity AS activity
JOIN pg_stat_activity AS blocking ON blocking.pid = ANY(pg_blocking_pids(activity.pid));

Check the duration of blocking caused and also identified the table locked.

SELECT w.query                          AS waiting_query,
       w.pid                            AS waiting_pid,
       w.usename                        AS waiting_user,
       now() - w.query_start            AS waiting_duration,
       l.query                          AS locking_query,
       l.pid                            AS locking_pid,
       l.usename                        AS locking_user,
       t.schemaname || '.' || t.relname AS tablename,
       now() - l.query_start            AS locking_duration
FROM pg_stat_activity w
         JOIN pg_locks l1 ON w.pid = l1.pid AND NOT l1.granted
         JOIN pg_locks l2 ON l1.relation = l2.relation AND l2.granted
         JOIN pg_stat_activity l ON l2.pid = l.pid
         JOIN pg_stat_user_tables t ON l1.relation = t.relid

You can terminate all blocking queries:

SELECT pg_cancel_backend(a.pid)
FROM( 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) a

You can also remove blocking by terminating the sessions:

SELECT  pg_terminate_backend(a.pid)
FROM( 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) a