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Re: Curious about dead rows.

From: Erik Jones <erik(at)myemma(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Russell Smith <mr-russ(at)pws(dot)com(dot)au>, Jean-David Beyer <jeandavid8(at)verizon(dot)net>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Curious about dead rows.
Date: 2007-11-15 16:02:36
Message-ID: 91BAAA02-1019-4FB1-AF2B-38BEDB32298C@myemma.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Nov 14, 2007, at 4:46 PM, Tom Lane wrote:

> Russell Smith <mr-russ(at)pws(dot)com(dot)au> writes:
>> It is possible that analyze is not getting the number of dead rows  
>> right?
>
> Hah, I think you are on to something.  ANALYZE is telling the truth
> about how many "dead" rows it saw, but its notion of "dead" is "not  
> good
> according to SnapshotNow".  Thus, rows inserted by a not-yet-committed
> transaction would be counted as dead.  So if these are background
> auto-analyzes being done in parallel with inserting transactions that
> run for awhile, seeing a few not-yet-committed rows would be
> unsurprising.

Wouldn't this result in a variable number of dead rows being reported  
on separate runs including zero while no pending inserts are  
happening?  This may be a good way to verify that this is what is  
happening if he can quiet down his app long enough to run an ANALYZE  
in isolation.  Perhaps, if the ANALYZE runs fast enough he can just  
lock the table for the run.

Erik Jones

Software Developer | Emma®
erik(at)myemma(dot)com
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