Re: truncate table getting blocked

Jayadevan M wrote:
> I have a python script. It opens a cursor, and sets the search_path (using psycopg2). In case
> something goes wrong in the script , a record is inserted into a table. In that script, I am not doing
> any thing else other than reading a file and publishing the lines to a queue (no database operations).
> The database is used just to track the errors. But my set search_path is locking a truncate table I am
> executing from a psql session. Is this expected?
>
> When the truncate table hung, I used this query
> 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;
>
> and got this (schemaname/user/tablename modified)-
>
> blocked_pid | blocked_user | blocking_pid | blocking_user | blocked_statement |
> current_statement_in_blocking_process
> -------------+--------------+--------------+---------------+----------------------------------+-------
> --------------------------------
> 9223 | myuser | 12861 | myuser | truncate table myschema.table1; | SET
> search_path TO myschema,public
>
>
> PG version :
>
> PostgreSQL 9.4.5 on x86_64-suse-linux-gnu, compiled by gcc (SUSE Linux) 4.8.5, 64-bit

It is not the "SET search_path" statement that is blocking the truncate,
but probably some earlier statement issued in the same transaction.

Take a look at pg_locks to find out what lock the transaction is holding on myschema.table1.

Use statement logging to find out which statement causes the lock.

Yours,
Laurenz Albe