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

User · Answer

Postgres has a very rich system catalog exposed via SQL tables. PG's statistics collector is a subsystem that supports collection and reporting of information about server activity.

Now to figure out the blocking PIDs you can simply query pg_stat_activity.

select pg_blocking_pids(pid) as blocked_by
from pg_stat_activity
where cardinality(pg_blocking_pids(pid)) > 0;

To, get the query corresponding to the blocking PID, you can self-join or use it as a where clause in a subquery.

SELECT query
FROM pg_stat_activity
WHERE pid IN (select unnest(pg_blocking_pids(pid)) as blocked_by from pg_stat_activity where cardinality(pg_blocking_pids(pid)) > 0);

Note: Since pg_blocking_pids(pid) returns an Integer[], so you need to unnest it before you use it in a WHERE pid IN clause.

Hunting for slow queries can be tedious sometimes, so have patience. Happy hunting.

User · Answer

One thing I find that is often missing from these is an ability to look up row locks   At least on the larger databases I have worked on  row locks are not shown in pg locks  if they were  pg locks would be much  much larger and there isn t a real data type to show the locked row in that view properly    I don t know that there is a simple solution to this but usually what I do is look at the table where the lock is waiting and search for rows where the xmax is less than the transaction id present there   That usually gives me a place to start  but it is a bit hands-on and not automation friendly   Note that shows you uncommitted writes on rows on those tables   Once committed  the rows are not visible in the current snapshot   But for large tables  that is a pain

User · Answer

From this excellent article on query locks in Postgres  one can get blocked query and blocker query and their information from the following query    CREATE VIEW lock monitor AS  SELECT   COALESCE blockingl relation  regclass  text blockingl locktype  as locked item    now   - blockeda query start AS waiting duration  blockeda pid AS blocked pid    blockeda query as blocked query  blockedl mode as blocked mode    blockinga pid AS blocking pid  blockinga query as blocking query    blockingl mode as blocking mode FROM pg catalog pg locks blockedl JOIN pg stat activity blockeda ON blockedl pid   blockeda pid JOIN pg catalog pg locks blockingl ON       blockingl transactionid blockedl transactionid  OR    blockingl relation blockedl relation AND blockingl locktype blockedl locktype      AND blockedl pid    blockingl pid  JOIN pg stat activity blockinga ON blockingl pid   blockinga pid   AND blockinga datid   blockeda datid WHERE NOT blockedl granted AND blockinga datname   current database       SELECT   from lock monitor    As the query is long but useful  the article author has created a view for it to simplify it s usage

User · Answer

This modification of a horse with no name s answer will give you the blocking queries in addition to just the blocked sessions   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

User · Answer

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    gt  0

[sql] How to detect query which holds the lock in Postgres?

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