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)
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