From: Tom Lane <tgl@sss.pgh.pa.us>
To: Egor Rogov <e.rogov@postgrespro.ru>
cc: pgsql-docs@postgresql.org
Subject: Re: FrozenTransactionId
In-reply-to: <57CF23F9.10304@postgrespro.ru>
References: <575D7955.6060209@postgrespro.ru>
 <1473096445753-5919545.post@n3.nabble.com> <31833.1473097584@sss.pgh.pa.us>
 <57CDED18.8060005@postgrespro.ru> <57CE6F15.4060101@postgrespro.ru>
 <16060.1473165694@sss.pgh.pa.us> <57CF23F9.10304@postgrespro.ru>
Comments: In-reply-to Egor Rogov <e.rogov@postgrespro.ru>
	message dated "Tue, 06 Sep 2016 23:15:53 +0300"
Date: Tue, 06 Sep 2016 16:41:39 -0400
Message-ID: <18782.1473194499@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-docs-owner@postgresql.org

Egor Rogov <e.rogov@postgrespro.ru> writes:
> Right, it does not say that FrozenTransactionId is what appears on disk, 
> but what is? There is no such information anywhere in the doc. Since 9.4 
> frozen transactions have their normal XIDs preserved, so how a user can 
> tell normal transaction from frozen one? This is what needs to be 
> explained, I believe.

I'm afraid the answer is "you can't tell".  The infomask bits in tuple
headers aren't exposed via SQL.  If you're really desperate,
contrib/pageinspect would help, but I don't propose mentioning that here.

In general, I'm not really sure why users would care very much at
a tuple-by-tuple level.  Aggregate statistics would be interesting,
which raises the question why contrib/pgstattuple doesn't provide
frozen-tuples counts.

			regards, tom lane


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