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Subject: Re: unusual performance for vac following 8.2 upgrade
From: "Simon Riggs" <simon@2ndquadrant.com>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
Cc: "Kim" <kim@myemma.com>, <pgsql-performance@postgresql.org>,
	<pgsql-hackers@postgresql.org>
In-Reply-To: <28949.1168544710@sss.pgh.pa.us>
References: <45A671CD.1050706@myemma.com> <27939.1168539652@sss.pgh.pa.us>
	<45A68C74.4080405@myemma.com> <28949.1168544710@sss.pgh.pa.us>
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Date: Thu, 11 Jan 2007 20:40:06 +0000
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On Thu, 2007-01-11 at 14:45 -0500, Tom Lane wrote:
> Kim <kim@myemma.com> writes:
> > We were running on 8.1.1 previous to upgrading to 8.2, and yes, we 
> > definitely have a heafty pg_class. The inheritance model is heavily used 
> > in our schema (the results of the group by you wanted to see are down 
> > below).  However, no significant problems were seen with vacs while we 
> > were on 8.1.
> 
> Odd, because the 8.1 code looks about the same, and it is perfectly
> obvious in hindsight that its runtime is about O(N^2) in the number of
> relations :-(.  At least that'd be the case if the stats collector
> output were fully populated.  Did you have either stats_block_level or
> stats_row_level turned on in 8.1?  If not, maybe the reason for the
> change is that in 8.2, that table *will* be pretty fully populated,
> because now it's got a last-vacuum-time entry that gets made even if the
> stats are otherwise turned off.  Perhaps making that non-disablable
> wasn't such a hot idea :-(.
> 
> What I think we need to do about this is
> 
> (1) fix pgstat_vacuum_tabstats to have non-O(N^2) behavior; I'm thinking
> of using a hash table for the OIDs instead of a linear list.  Should be
> a pretty small change; I'll work on it today.
> 
> (2) Reconsider whether last-vacuum-time should be sent to the collector
> unconditionally.
> 
> Comments from hackers?

It's not clear to me how this fix will alter the INSERT issue Kim
mentions. Are those issues connected? Or are you thinking that handling
stats in a tight loop is slowing down other aspects of the system?

-- 
  Simon Riggs             
  EnterpriseDB   http://www.enterprisedb.com