From: Bruce Momjian <pgman@candle.pha.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: josh@agliodbs.com, Manfred Koizar <mkoi-pg@aon.at>
Cc: Aaron Held <aaron@MetroNY.com>
Cc: Roberto Mello <rmello@cc.usu.edu>
Cc: Neil Conway <neilc@samurai.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: [GENERAL] CURRENT_TIMESTAMP
Date: Mon, 23 Sep 2002 20:11:26 -0400 (EDT)
Message-ID: <200209240011.g8O0BVL11014@candle.pha.pa.us> (raw)
In-Reply-To: <4619.1032814548@sss.pgh.pa.us>
I see what you are saying now --- that even single user statements can
trigger multiple statements, so you would have to say transaction start
time is time the user query starts. I can see how that seems a little
arbitrary. However, don't we have separate paths for user queries and
queries sent as part of a rule?
---------------------------------------------------------------------------
Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I don't see how we can defend returning the start of the transaction as
> > the current_timestamp.
>
> Here's an example:
>
> CREATE RULE foo AS ON INSERT TO mytable DO
> ( INSERT INTO log1 VALUES (... , now(), ...);
> INSERT INTO log2 VALUES (... , now(), ...) );
>
> I think it's important that these commands store the same timestamp in
> both log tables (not to mention that any now() being stored into mytable
> itself generate that same timestamp).
>
> If you scale that up just a little bit, you can devise scenarios where
> successive client-issued commands (within a single transaction) want to
> store the same timestamp. After all, it's only a minor implementation
> detail that you chose to fire these logging operations via a rule and
> not by client-side logic.
>
> In short, there are plenty of situations where it's critical for
> application correctness that a series of commands all be able to operate
> with the same value of now(). I don't think that it's wise for Postgres
> to try to decide where within a transaction it's safe to advance now().
> That will inevitably break some applications, and it's not obvious what
> the benefit is.
>
> In short: if you want exact current time, there's timeofday(). If you
> want start of transaction time, we've got that. If you want start of
> current statement time, I have two questions: why, and exactly how do
> you want to define current statement, considering functions, rules,
> triggers, and all that other stuff that makes it interesting?
>
> ISTM that if a client or function wants to record intratransaction
> times, it can call timeofday() at the appropriate points for itself.
>
> regards, tom lane
>
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
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To: pgsql-sql@postgresql.org
Cc: pgman@candle.pha.pa.us, tgl@sss.pgh.pa.us, mkoi-pg@aon.at, aaron@MetroNY.com, rmello@cc.usu.edu, neilc@samurai.com
Subject: Re: [GENERAL] CURRENT_TIMESTAMP
In-Reply-To: <200209240011.g8O0BVL11014@candle.pha.pa.us>
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