Date: Thu, 1 Jun 2017 15:16:40 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Mahesh Jhala <mjhala@carrentals.com>
Cc: "pgsql-admin@postgresql.org" <pgsql-admin@postgresql.org>
Subject: Re: Can manual vacuuming conflict with auto vacuuming?
Message-ID: <20170601191640.rj6zda65pd5tgjrg@alvherre.pgsql>
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Mahesh Jhala wrote:
> Hello All,
> 
> If autovacuum of a large PostgreSQL table is in progress, and I start
> a manual vacuum on the table, is there likely to be a conflict
> (locking etc.) between the two vacuums?

It depends on the autovacuum being for-wraparound or not.  If not, then
the conflict will be detected within one second and autovacuum will
cancel itself, so the regular vacuum would be able to proceed.  So there
is a one-second period of time during which you would see the manual
vacuum as blocked by autovacuum.

If autovacuum is for wraparound protection, then it won't cancel itself,
and the regular vacuum will just wait until autovacuum is finished.


There is a final stage in vacuuming (both auto and regular) during which
the table is truncated (the last few empty pages, if any).  There's some
heuristics to handle that case nowadays which I don't remember very
clearly.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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