Re: [PATCHES] NO WAIT ...

Tom Lane wrote:

> Bruce Momjian <pgman@candle.pha.pa.us> writes:
>> The question is whether we should have a GUC variable to control no
>> waiting on locks or add NO WAIT to specific SQL commands.
> 
> That's only a minor part of the issue.  The major problem I have with
> the patch is that it affects *all* locks, including system-internal
> lock attempts that the user is probably not even aware of much less
> able to control.  It's like giving someone a poorly-aligned shotgun
> when what they need is a rifle --- they'll end up putting holes in
> a lot of other things besides what they intended.

I absolutely agree to that.

> 
> I think that what we actually want is something that is narrowly
> tailored to affect only row-level locks taken by SELECT FOR UPDATE,
> and maybe one or two other places that (a) people can make specific
> use-cases for, and (b) we can be certain are only invoked by user
> commands and never indirectly from behind-the-scenes system operations.

I would gereralize this to user table row level and explicit lock table. 
There is no need to force a separate SELECT FOR UPDATE in front of every 
UPDATE or DELETE attempt in order to achieve the desired nolock behaviour.

> 
> The reason for proposing syntax rather than a GUC variable is the same
> one of control.  If you set a GUC variable then it will be hard to
> prevent it from breaking operations other than the one you thought you
> intended.  (Example: you think you are only causing your SELECT FOR
> UPDATE to error out, but what about ones done behind the scenes for
> foreign key checks?)  GUC variables are good for stuff that tends to
> apply application-wide, which is why I thought regex_flavor wasn't too
> dangerous, but they're terrible for functions that you want to apply to
> only certain specific operations.  And I can't imagine an app where that
> wouldn't be true for NO WAIT.

This all needs a lot more detail than it has so far. If one tries to 
UPDATE a row with NOLOCK, and a trigger fired during this would block on 
the attempt to create a temp table, I think the resulting transaction 
abort would rather be reported as a bug to us, then accepted as a nice 
feature.

If there is a call for vote, I vote against.


Jan

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