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Re: Additional current timestamp values

From: Neil Conway <neilc(at)samurai(dot)com>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: Brendan Jurd <direvus(at)gmail(dot)com>, pgsql-patches(at)postgresql(dot)org
Subject: Re: Additional current timestamp values
Date: 2006-03-20 23:05:02
Message-ID: 441F351E.8030203@samurai.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Bruce Momjian wrote:
>     <function>CURRENT_TIMESTAMP</> might not be the
>     transaction start time on other database systems.
>     For this reason, and for completeness,
>     <function>transaction_timestamp</> is provided.

Well, transaction_timestamp() is even more unlikely to be the 
transaction start time on other database systems :) If the user wants 
non-standard syntax for getting the timestamp at which the current 
transaction began, we already have now().

> One trick is that these should be the same:
> 
> 	test=> SELECT statement_timestamp(), transaction_timestamp();

Should they be? It seems quite reasonable to me that the DBMS begins a 
transaction internally (setting transaction_timestamp()), and then a 
short while later begins executing the statement submitted by the user, 
at which point statement_timestamp() is set.

Perhaps ensuring they are identical for single-statement transactions is 
the best behavior, I just don't think this is required behavior.

> And these should be the same:
> 
> 	$ psql -c '
> 	INSERT INTO t VALUES (statement_timestamp());
> 	INSERT INTO t VALUES (statement_timestamp());' test
> 	INSERT 0 1

Uh, why should these be the same?

-Neil

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