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Re: Phantom command ids again

From: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Phantom command ids again
Date: 2007-01-29 15:53:42
Message-ID: 45BE1886.9090103@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane wrote:
> Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
>> I was about to resubmit the phantom command ids patch for review, as I 
>> noticed a little problem.
> 
>> In fmgr.c in record_C_func, we cache the xmin and cmin, and later in 
>> lookup_C_func we check that they match to determine if the cached 
>> information is still valid. With phantom command ids, the cmin is not 
>> valid outside the inserting transaction, which makes it unusable for 
>> that purpose.
> 
> I think that actually that's just belt-and-suspenders programming;
> it should be sufficient to compare tuple TID and xmin.  AFAICS a single
> transaction cannot fill the same TID twice, since VACUUM would never
> dare remove a tuple entered by a still-in-progress transaction.  So the
> cmin check doesn't seem necessary.

We don't currently use tid in the up-to-dateness check. Just  Oid, xmin 
and cmin. Good point, tid would work. I'll change it do that in the patch.

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

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