On Mon, Oct 07, 2019 at 11:00:08PM +0200, Artur Zając wrote:
Hi,
I cannot reproduce some scenario I found in my PostgreSQL logs.
I have two connections/processes:
Process 24755 is standard pg_backup connection with:
.
BEGIN;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ, READ ONLY
.
LOCK TABLE gm.tableabc;
.
COPY FROM gm.tableabc
Process 5969 is normal connection where I have:
LOG: proces 5969 still waiting for AccessExclusiveLock on relations
562888531 of database 16429 after 1000.066 ms
DETAILT: Process holding the lock: 24755. Wait queue: 5969
Query that is waiting (from proces 5969) is:
CREATE TEMP TABLE IF NOT EXISTS tableabc
(
Id BIGINT DEFAULT gm.top()
) INHERITS (gm.tableabc);
I cannot reproduce what pg_dump is doing that causes waiting on proces 5969
(which lock is acquired and why). When pg_dump ends lock is released and
proces 5969 continues.
I know that doing in two parallel connections:
BEGIN;
CREATE TEMP TABLE IF NOT EXISTS tableabc
(
Id BIGINT DEFAULT gm.top()
) INHERITS (gm.tableabc);
causes waiting state on second connection until the first finished, but
pg_dump connection is read only.
Not sure why would it matter that the pg_dump connection is read-only,
this is about locking because pg_dump needs to ensure the schema does
not change while it's running.
pg_dump does not do
LOCK TABLE gm.tableabc;
but
LOCK TABLE gm.tableabc IN ACCESS SHARE MODE;
Which should be visible in pg_locks system view. And it does conflict
with the ACCESS EXCLUSIVE mode, used by the second query.
Could you suggest me which part of pg_dump (which query) might cause that
behaviour.
It's this line:
https://github.com/postgres/postgres/blob/master/src/bin/pg_dump/pg_dump.c#L6676
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services