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Re: VACUUM ANALYZE blocking both reads and writes to atable

From: Peter Schuller <peter(dot)schuller(at)infidyne(dot)com>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: VACUUM ANALYZE blocking both reads and writes to atable
Date: 2008-06-30 15:43:18
Message-ID: 20080630154318.GB15756@hyperion.scode.org (view raw or flat)
Thread:
Lists: pgsql-performance
Actually, while on the topic:

>     date: 2007-09-10 13:58:50 -0400;  author: alvherre;  state: Exp;  lines: +6 -2;
>     Remove the vacuum_delay_point call in count_nondeletable_pages, because we hold
>     an exclusive lock on the table at this point, which we want to release as soon
>     as possible.  This is called in the phase of lazy vacuum where we truncate the
>     empty pages at the end of the table.

Even with the fix the lock is held. Is the operation expected to be
"fast" (for some definition of "fast") and in-memory, or is this
something that causes significant disk I/O and/or scales badly with
table size or similar?

I.e., is this enough that, even without the .4 bug, one should not
really consider VACUUM ANALYZE non-blocking with respect to other
transactions?

(I realize various exclusive locks are taken for short periods of time
even for things that are officially declared non-blocking; the
question is whether this falls into this category.)

-- 
/ Peter Schuller

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