From: Bruce Momjian <bruce@momjian.us>
Message-Id: <200705161515.l4GFF9T03064@momjian.us>
Subject: Re: [HACKERS] row-level stats and last analyze time
In-Reply-To: <18709.1178501121@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 16 May 2007 11:15:09 -0400 (EDT)
CC: Neil Conway <neilc@samurai.com>,
	Guillaume Lelarge <guillaume@lelarge.info>, pgsql-docs@postgresql.org,
	pgsql-hackers <pgsql-hackers@postgresql.org>
MIME-Version: 1.0
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Has this been done yet?  I don't think so.

---------------------------------------------------------------------------

Tom Lane wrote:
> Neil Conway <neilc@samurai.com> writes:
> > On Thu, 2007-26-04 at 18:07 -0400, Neil Conway wrote:
> >> (1) I believe the reasoning for Tom's earlier change was not to reduce
> >> the I/O between the backend and the pgstat process [...]
> 
> > Tom, any comments on this? Your change introduced an undocumented
> > regression into 8.2. I think you're on the hook for a documentation
> > update at the very least, if not a revert.
> 
> The documentation update seems the most prudent thing to me.  The
> problem with the prior behavior is that it guarantees that every table
> in the database will eventually have a pg_stat entry, even if
> stats_row_level and stats_block_level are both off.  In a DB with lots
> of tables that creates a significant overhead *for a feature the DBA
> probably thinks is turned off*.  This is not how it worked before 8.2,
> and so 8.2.0's behavior is arguably a performance regression compared
> to 8.1 and before.
> 
> Now this patch went in before we realized that 8.2.x had a bug in
> computing the stats-file-update delay, and it could be that after fixing
> that the problem is not so pressing.  But I don't particularly care for
> new features that impose a performance penalty on those who aren't using
> them, and that's exactly what last_vacuum/last_analyze tracking does if
> we allow it to bloat the stats file in the default configuration.
> 
> The long-term answer of course is to devise a more efficient stats
> reporting scheme, but I'm not sure offhand what that would look like.
> 
> 			regards, tom lane
> 
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-- 
  Bruce Momjian  <bruce@momjian.us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +