From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Bruce Momjian <bruce@momjian.us>,
        Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
        Michael Paquier <michael@paquier.xyz>,
        Andres Freund <andres@anarazel.de>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: transction_timestamp() inside of procedures
In-reply-to: <20180926141336.z7gpojck3tpruqwv@alvherre.pgsql>
References: <20180926141336.z7gpojck3tpruqwv@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
	message dated "Wed, 26 Sep 2018 11:13:36 -0300"
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Date: Wed, 26 Sep 2018 10:36:01 -0400
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> On 2018-Sep-26, Tom Lane wrote:
>> I agree that it would be surprising for transaction timestamp to be newer
>> than statement timestamp.  So for now at least, I'd be satisfied with
>> documenting the behavior.

> Really?  I thought it was practically obvious that for transaction-
> controlling procedures, the transaction timestamp would not necessarily
> be aligned with the statement timestamp.  The surprise would come
> together with the usage of the new feature, so existing users would not
> be surprised in any way.

Nope.  That's the same poor reasoning we've fallen into in some other
cases, of assuming that "the user" is a point source of knowledge.
But DBMSes tend to interact with lots of different code.  If some part
of application A starts using intraprocedure transactions, and then
application B breaks because it wasn't expecting to see xact_start
later than query_start in pg_stat_activity, you've still got a problem.

I'm also a bit hesitant to invent new semantics here based on the
assumption that we've got only one, nonoverlapping, top-level transaction
at a time.  It's not terribly hard to imagine suspend-and-resume-
transaction features coming down the pike at some point.  What will
we do then?  We'll already have a definitional issue for xact_start,
but it'll get worse the more different kinds of xact_start we have.

			regards, tom lane