1) In config:
shared_preload_libraries = 'pg_stat_statements'
2) After it restart the node via using `service postgresql restart`
3) In psql:
CREATE EXTENSION pg_stat_statements;

Queries:
SELECT pid,datname,usename,client_addr,query_start,query FROM pg_stat_activity;

# limit of slow queries
SELECT substring(query, 1, 50) AS short_query,
       round(total_time::numeric, 2) AS total_time,
       calls,
       round(mean_time::numeric, 2) AS mean,
       round((100 * total_time /
       sum(total_time::numeric) OVER ())::numeric, 2) AS percentage_cpu
FROM    pg_stat_statements
ORDER BY total_time DESC
LIMIT 20;