From: Holger Jakobs <holger@jakobs.com>
To: pgsql-admin@lists.postgresql.org
Subject: Re: now() and statement_timestamp()
Date: Fri, 28 May 2021 13:15:54 +0200
Message-ID: <eb09bcc5-4676-a5fc-2d0b-c165baf16264@jakobs.com> (raw)
In-Reply-To: <1785725.1622133836@sss.pgh.pa.us>
References: <536812b3-8040-371d-506d-266bcf10ec2f@jakobs.com>
<1785725.1622133836@sss.pgh.pa.us>
Am 27.05.21 um 18:43 schrieb Tom Lane:
> Holger Jakobs <holger@jakobs.com> writes:
>> So in psql now() and statement_timestamp() are identical when not in a
>> transaction, while in the others the two values vary a little bit,
>> roughly 100 microseconds.
>> How come the same statements aren't behaving identical. I'm using a
>> current PostgreSQL server on a Linux machine.
> I'd try turning on log_statement = all to see what's actually
> happening. What seems somewhat likely is that the non-psql
> cases are wrapping the command you gave in BEGIN/COMMIT,
> or something along that line.
>
> [ thinks for a bit ] It could also be down to the wire protocol
> used. psql is just sending a simple Query message, but the
> other two might well be sending Parse/Bind/Execute. IIRC the
> transaction must be started by Bind, but we may consider
> the statement timestamp to be the start of processing Execute.
>
> regards, tom lane
I turned on logging of statements, and found out that psql uses a simple
query, while the others us an extended query.
According to the documentation a simple query and a series of parse,
bind, execute are approximately equivalent (Note right above "Function
Call" in
https://www.postgresql.org/docs/13/protocol-flow.html#id-1.10.5.7.6)
So actually the "approximately equivalent" is the point here. The series
of parse, bind, execute doesn't call the two funtions now() and
statement_timestamp() at the same stage, so the timestamps vary.
I wasn't aware of this effect. Now it's clear that one can only use a
simple query in order to find out whether a session is within a
transaction or in autocommit mode. Maybe the Note in the documentation
should mention this clearly.
--
Holger Jakobs, Bergisch Gladbach, Tel. +49-178-9759012
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To: pgsql-admin@postgresql.org
Cc: holger@jakobs.com, pgsql-admin@lists.postgresql.org
Subject: Re: now() and statement_timestamp()
In-Reply-To: <eb09bcc5-4676-a5fc-2d0b-c165baf16264@jakobs.com>
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