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Date: Thu, 11 May 2006 20:03:19 +0200
From: Martijn van Oosterhout <kleptog@svana.org>
To: "Jim C. Nasby" <jnasby@pervasive.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, PFC <lists@peufeu.com>,
	Mitchell Skinner <mitch@arctur.us>,
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Subject: Re: [HACKERS] Big IN() clauses etc : feature proposal
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On Thu, May 11, 2006 at 12:18:06PM -0500, Jim C. Nasby wrote:
> > Yes, because there can be more than one active snapshot within a single
> > transaction (think about volatile functions in particular).
>=20
> Any documentation on how snapshot's work? They're a big mystery to me.
> :(

A snapshot is a particular view on a database. In particular, you have
to be able to view a version of the database that doesn't have you own
changes, otherwise an UPDATE would keep updating the same tuple. Also,
for example, a cursor might see an older version of the database than
queries being run. I don't know of any particular information about it
though. Google wasn't that helpful.

> > No; you forgot about subtransactions.
>=20
> Oh, I thought those were done with cmin and cmax... if that's not what
> cmin/cmax are for, then what is?

cmin/cmax are command counters. So in the sequence:

BEGIN;
SELECT 1;
SELECT 2;

The second query runs as the same transaction ID but a higher command
ID so it can see the result of the previous query. Subtransactions are
(AIUI anyway) done by having transactions depend on other transactions.
When you start a savepoint you start a new transaction ID whose status
is tied to its top-level transaction ID but can also be individually
rolledback.

Have a nice day,
--=20
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to l=
itigate.

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