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Re: COMMIT NOWAIT Performance Option

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: COMMIT NOWAIT Performance Option
Date: 2007-02-27 04:04:07
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Lists: pgsql-hackers
"Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> COMMIT NOWAIT can co-exist with the normal form of COMMIT and does not
> threaten the consistency or robustness of other COMMIT modes. Read that
> again and think about it, before we go further, please.

I read that, and thought about it, and don't think I believe it.  The
problem is that there are more interconnections between different
transactions than you're allowing for.  In particular you need to
justify that the behavior is safe for non-transactional operations like
btree page splits and pg_clog buffer page writes.  The idea that's
particularly bothering me at the moment is that after a system crash,
we might come back up in a state where a NOWAIT transaction appears
committed when its updates didn't all get to disk.  "Database corrupt"
is a situation that threatens all your transactions...

> New commit mode is available by explicit command, or as a default
> setting that will be applied to all COMMITs, or both.

I dislike introducing new nonstandard syntax ("Oracle compatible" is not
standard).  If we did this I'd vote for control via a GUC setting only;
I think that is more useful anyway, as an application can be made to run
with such a setting without invasive source code changes.

			regards, tom lane

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Subject: Re: COMMIT NOWAIT Performance Option
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Subject: Re: autovacuum next steps, take 2

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