[GENERAL] truncate table getting blocked

[Jayadevan M]

Hello,

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


Thanks,
Jayadevan