Обсуждение: Can statement_timeout emulated NOWAIT?

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
issimple 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
becauseit will track timeouts on a row by row basis, which eliminates false positives.  That is, if the
statement_timeoutis too short for the select, it might fail before finishing the select instead of failing because of
lockedrow.
 

- 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   

"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

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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Excellent! Thanks for your prompt replies.

--
Luis P Caamano
Atlanta, GA, USA