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* Can statement_timeout emulated NOWAIT?
@ 2004-03-16 13:58 Luis P Caamano <lcaamano@mindspring.com>
0 siblings, 2 replies; 4+ messages in thread
From: Luis P Caamano @ 2004-03-16 13:58 UTC (permalink / raw)
To: pgsql-sql
I've researched the recent discussions about statement_timeout
and NOWAIT and I'd like to confirm my understanding of the
situation before acting on it. Please let me know if the
following statements are true:
- LOCK TABLE ... NOWAIT has been checked in and will be available
in 7.5
- SELECT ... FOR UPDATE NOWAIT is in the TODO list (among other
NOWAIT commands) but it will also be available in 7.5
- Using "SET LOCAL statement_timeout=xxx" can be used before
"SELECT ... FOR UPDATE" to emulate NOWAIT when the select
is simple and involves a relatively small number of rows.
That is, the following:
BEGIN;
SET LOCAL statement_timeout=3000;
SELECT ... FOR UPDATE; -- (1 row)
COMMIT;
will be equivalent to:
BEGIN;
SELECT ... FOR UPDATE NOWAIT;
COMMIT;
- The NOWAIT feature will be able to handle select-for-updates
that involve many rows better than statement_timeout because
it will track timeouts on a row by row basis, which eliminates
false positives. That is, if the statement_timeout is too
short for the select, it might fail before finishing the select
instead of failing because of locked row.
- The error raised by statement_timeout and NOWAIT will be the
same.
Basically, I'd like to write low level code that implements
NOWAIT in the application using statement_timeout. Later on,
7.5, I'd be able to reimplement the low level functions using
NOWAIT without having to change the higher level code.
Your thoughts and comments are appreciated.
Thanks
Luis P Caamano
Atlanta, GA, USA
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Can statement_timeout emulated NOWAIT?
@ 2004-03-16 15:48 Tom Lane <tgl@sss.pgh.pa.us>
parent: Luis P Caamano <lcaamano@mindspring.com>
1 sibling, 1 reply; 4+ messages in thread
From: Tom Lane @ 2004-03-16 15:48 UTC (permalink / raw)
To: Luis P Caamano <lcaamano@mindspring.com>; +Cc: pgsql-sql
"Luis P Caamano" <lcaamano@mindspring.com> writes:
> I've researched the recent discussions about statement_timeout
> and NOWAIT and I'd like to confirm my understanding of the
> situation before acting on it. Please let me know if the
> following statements are true:
> - LOCK TABLE ... NOWAIT has been checked in and will be available
> in 7.5
Yes.
> - SELECT ... FOR UPDATE NOWAIT is in the TODO list (among other
> NOWAIT commands) but it will also be available in 7.5
I haven't seen anyone working on that.
> - Using "SET LOCAL statement_timeout=xxx" can be used before
> "SELECT ... FOR UPDATE" to emulate NOWAIT when the select
> is simple and involves a relatively small number of rows.
Yes, if you are sure you know how long the select "ought" to take.
> - The error raised by statement_timeout and NOWAIT will be the
> same.
Wouldn't expect so. (Right now, I think statement_timeout reports
the same error code as query cancel, which is pretty bogus also.)
regards, tom lane
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Can statement_timeout emulated NOWAIT?
@ 2004-03-16 15:56 Bruce Momjian <pgman@candle.pha.pa.us>
parent: Luis P Caamano <lcaamano@mindspring.com>
1 sibling, 0 replies; 4+ messages in thread
From: Bruce Momjian @ 2004-03-16 15:56 UTC (permalink / raw)
To: Luis P Caamano <lcaamano@mindspring.com>; +Cc: pgsql-sql
Yes, I think these are all correct, except I am not positive FOR UPDATE
NOWAIT will be in 7.5 unless someone codes it.
---------------------------------------------------------------------------
Luis P Caamano wrote:
>
> I've researched the recent discussions about statement_timeout
> and NOWAIT and I'd like to confirm my understanding of the
> situation before acting on it. Please let me know if the
> following statements are true:
>
> - LOCK TABLE ... NOWAIT has been checked in and will be available
> in 7.5
>
> - SELECT ... FOR UPDATE NOWAIT is in the TODO list (among other
> NOWAIT commands) but it will also be available in 7.5
>
> - Using "SET LOCAL statement_timeout=xxx" can be used before
> "SELECT ... FOR UPDATE" to emulate NOWAIT when the select
> is simple and involves a relatively small number of rows.
> That is, the following:
>
> BEGIN;
> SET LOCAL statement_timeout=3000;
> SELECT ... FOR UPDATE; -- (1 row)
> COMMIT;
>
> will be equivalent to:
>
> BEGIN;
> SELECT ... FOR UPDATE NOWAIT;
> COMMIT;
>
> - The NOWAIT feature will be able to handle select-for-updates
> that involve many rows better than statement_timeout because
> it will track timeouts on a row by row basis, which eliminates
> false positives. That is, if the statement_timeout is too
> short for the select, it might fail before finishing the select
> instead of failing because of locked row.
>
> - The error raised by statement_timeout and NOWAIT will be the
> same.
>
> Basically, I'd like to write low level code that implements
> NOWAIT in the application using statement_timeout. Later on,
> 7.5, I'd be able to reimplement the low level functions using
> NOWAIT without having to change the higher level code.
>
> Your thoughts and comments are appreciated.
>
> Thanks
>
> Luis P Caamano
> Atlanta, GA, USA
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 7: don't forget to increase your free space map settings
>
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Can statement_timeout emulated NOWAIT?
@ 2004-03-16 16:33 Luis P Caamano <lcaamano@mindspring.com>
parent: Tom Lane <tgl@sss.pgh.pa.us>
0 siblings, 0 replies; 4+ messages in thread
From: Luis P Caamano @ 2004-03-16 16:33 UTC (permalink / raw)
To: pgsql-sql
Excellent! Thanks for your prompt replies.
--
Luis P Caamano
Atlanta, GA, USA
^ permalink raw reply [nested|flat] 4+ messages in thread
end of thread, other threads:[~2004-03-16 16:33 UTC | newest]
Thread overview: 4+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2004-03-16 13:58 Can statement_timeout emulated NOWAIT? Luis P Caamano <lcaamano@mindspring.com>
2004-03-16 15:48 ` Tom Lane <tgl@sss.pgh.pa.us>
2004-03-16 16:33 ` Luis P Caamano <lcaamano@mindspring.com>
2004-03-16 15:56 ` Bruce Momjian <pgman@candle.pha.pa.us>
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