Just a quick footnote: If autovac truncations are frequently causing replica lag, and if this is a problem for you, IIUC one way you can stop autovac from doing the truncations even on older versions is setting old_snapshot_threshold to any value at all besides zero. (On 12+ you can directly control the truncation behavior.)
-Jeremy
Sent from my TI-83
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
Right, that part I understood after checking out pg_attribute's insert/delete counts in pg_stat_sys_tables before and after some REFRESH MATERIALIZED VIEW runs on an otherwise idle server. With them running 2k+ refreshes per day autovac is regularly working on their catalog tables.
Thanks!
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