From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200403161556.i2GFusW22068@candle.pha.pa.us>
Subject: Re: Can statement_timeout emulated NOWAIT?
In-Reply-To: <DIEAJDKECNMGBBCLCGFFAEHPCGAA.lcaamano@mindspring.com>
To: Luis P Caamano <lcaamano@mindspring.com>
Date: Tue, 16 Mar 2004 10:56:54 -0500 (EST)
Cc: pgsql-sql@postgresql.org
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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
>     
> 
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