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