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Re: WIP fix proposal for bug #6123

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Florian Pflug <fgp(at)phlo(dot)org>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: WIP fix proposal for bug #6123
Date: 2011-08-03 15:57:12
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Lists: pgsql-hackers
On Wed, Aug 3, 2011 at 4:50 AM, Florian Pflug <fgp(at)phlo(dot)org> wrote:
> On Aug3, 2011, at 04:54 , Robert Haas wrote:
>> On Tue, Aug 2, 2011 at 2:32 PM, Kevin Grittner
>> <Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
>>>>> Would you feel comfortable with a patch which threw an error on
>>>>> the DELETE case, as it does on the UPDATE case?
>>>> Yes, though with a little additional twist. The twist being that
>>>> I'd like the error to be thrown earlier, at the time of the
>>>> offending UPDATE or DELETE, not later, when the original UPDATE or
>>>> DELETE which caused the BEFORE trigger invocation stumbles over
>>>> the updated row. It not only seems more logical that way, but it
>>>> also makes it possible for the trigger to catch the error, and
>>>> react accordingly.
>>> I hadn't thought of that.  It does seem better in every way except
>>> for how much work it takes to do it and how much testing it needs to
>>> have confidence in it.  Definitely not 9.1 material.
>> I'm not sure I understand the justification for throwing an error.
>> Updating a row twice is not an error under any other circumstances;
>> nor is deleting a row you've updated.  Why should it be here?
> Because updating or deleting a row from within a BEFORE trigger fired
> upon updating or deleting that very row causes two intermingled
> UPDATE/DELETE executions, not one UPDATE/DELETE followed by another.
> Here's a step-by-step description of what happens in the case of two
> UPDATES, assuming that we don't ignore any updated and throw no error.
> 2) Row with ID=1 is found & locked
> 3) BEFORE UPDATE triggers are fired, with OLD containing the original
>   row's contents and NEW the values specified in (1)
> 4) UPDATE T SET ... WHERE ID=1, executed from within one of the triggers
> 5) BEFORE UPDATE triggers are fired again, with OLD containing the
>   original row's contents and NEW the value specified in (4).
> 6) The row is updated with the values specified in (4), possibly changed
>   by the triggers in (5)
> 7) The row is updated with the values specified in (2), possibly changed
>   by the triggers in (3).
> Now, in step (7), we're overwriting the UPDATE from steps 4-6 without
> even looking at the row that UPDATE produced. If, for example, both UPDATES
> do "counter = counter + 1", we'd end up with the counter incremented by
> 1, *not* by 2, even though there were two distinct UPDATEs. Or even worse,
> the inner UPDATE at (4) might have modified the ID. Then, we'll apply the
> outer UPDATE in (7), even though the original WHERE condition from (1)
> no longer matches the row.
> We could attempt to fix that by using the EvalPlanQual machinery to
> re-check the WHERE clause and re-compute the new row in (7). However,
> EvalPlanQual introduces a lot of conceptional complexity, and can lead
> to very surprising results for more complex UPDATES. (There's that whole
> self-join problem with EvalPlanQual, for example)
> Also, even if we did use EvalPlanQual, we'd still have executed the outer
> BEFORE trigger (step 3) with values for OLD and NEW which *don't* match
> the row the we actually update in (7). Doing that seems rather hazardous
> too - who knows what the BEFORE trigger has used the values for.
> The different between Kevin's original patch and my suggestion is, BTW,
> that he made step (7) through an error while I suggested the error to
> be thrown already at (4).

I think Kevin's proposal is better, because AFAICS if the BEFORE
UPDATE trigger returns NULL then we haven't got a problem; and we
can't know that until we get to step 7.

Robert Haas
The Enterprise PostgreSQL Company

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