Обсуждение: Hot Standby Conflict on pg_attribute

Hi,

On 2019-05-09 13:03:50 -0700, Erik Jones wrote:
> The question then is: Why would these user queries be waiting on an
> AccessShare lock on pg_attribute?  Thus far we've been unable to recreate
> any transacitons with the above query (and others) that show any
> pg_attribute locks.  There is no ORM in play here and these queries are
> being sent as single query transactions via this Node.js postgres adapter:
> https://github.com/brianc/node-postgres which is pretty bare bones.

Queries that access a table for the *first* time after DDL happened
(including truncating the relation), need an AccessShareLock on
pg_attribute (and pg_class, pg_index, ...) for a short time.

You can reproduce that fairly easily:

S1: CREATE TABLE foo();
S2: BEGIN; LOCK pg_attribute;
S1: SELECT * FROM foo;
S2: COMMIT;

S1 could execute the select, because it has a cached view of the way the
relation looks.

S2: ALTER TABLE foo ADD COLUMN bar INT;
S2: BEGIN; LOCK pg_attribute;
S1: SELECT * FROM foo;

Here S1 is blocked, because it needs to look at pg_attribute to figure
out the "shape" of the table, but it's currently locked.

Greetings,

Andres Freund

Andres Freund <andres@anarazel.de> writes:
> On 2019-05-09 13:03:50 -0700, Erik Jones wrote:
>> The question then is: Why would these user queries be waiting on an
>> AccessShare lock on pg_attribute?

> Queries that access a table for the *first* time after DDL happened
> (including truncating the relation), need an AccessShareLock on
> pg_attribute (and pg_class, pg_index, ...) for a short time.

Also, it seems likely that what's really triggering the issue is
autovacuum on pg_attribute trying to truncate off empty pages
in pg_attribute (after a bunch of dead rows were generated there
by DDL activity).  That requires exclusive lock on pg_attribute,
which would propagate down to the standby.

            regards, tom lane