Re: cancelled queries on standby

On 11/15/2011 05:46 AM, MirrorX wrote:
> i am facing an issue concerning cancelled queries made on a hot standby
> server. if we suppose that there are some tables that are never changed (no
> inserts/updates/deletes) and that autovacuum is set to false for these
> tables, what else could there be affecting these tables and consequently
> cancelling my queries? thx in advance for any suggestions
>

Query cancellation isn't targeted this way. Whether or not a query
needs to be canceled by vacuum cleanup depends on whether any open
transaction can see the rows that are being cleaned up. I think you
have the understandable belief what rows are actually being viewed by
the query running on the standby matter; they don't.

Let's say you have a long-running query Q accessing a table A on the
standby. Then some vacuum cleanup of table B happens on the master. Q
can then be canceled when those cleanup records are replicated. The
fact that A is never changed and doesn't have autovacuum enabled doesn't
matter. Because it's possible for Q to see B, it's canceled regardless
of whether it's looked at it yet.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us