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Re: Recalculating OldestXmin in a long-running vacuum

From: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Bruce Momjian <bruce(at)momjian(dot)us>, Gregory Stark <gsstark(at)mit(dot)edu>, pgsql-patches(at)postgresql(dot)org, ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
Subject: Re: Recalculating OldestXmin in a long-running vacuum
Date: 2007-02-23 12:15:36
Message-ID: 45DEDAE8.8020806@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-patches
Alvaro Herrera wrote:
> I'm wondering if there has been any effort to make this work in
> conjunction with ITAGAKI Takahiro's patch to correct the dead tuple
> count estimate.

I just looked at that patch. If we applied both patches, the dead_tuples 
estimate would be off by the number of dead tuples removed thanks to my 
patch.

In vacuum, we could count separately the tuples that were vacuumable 
according to the first snapshot, and tuples that were vacuumable 
according to a new snapshot. We could then get an estimate that's as 
good as with just Takahiro's patch with this formula:

new_n_dead_tuples = n_dead_tuples_at_end - (n_dead_tuples_at_start + 
tuples_removed_thanks_to_new_snapshot)

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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