Re: [BUGS] [HACKERS] Re: Postgresql bug report - unexpected behavior of suppress_redundant_updates_trigger

[Chapman Flack]

On 06/19/2017 05:19 PM, David G. Johnston wrote:

> At first glance I think I'd rather have it do the correct thing all of
> the time, even if it takes longer, so that my only trade-off decision
> is whether to improve performance by fixing the application.
> 
> Ideally if the input tuple wouldn't require compression we wouldn't
> bother to decompress the stored tuple.

That looks like one reasonable elimination check.

I wonder how much closer it could get with some changes that wouldn't
necessarily use many more cycles.

One might be a less_easy queue; marching through the tuple
comparing fields, if one is found to be TOASTed, throw it
on the queue and march on. Only if all the easy ones matched
is there any point in looking at the queue.

At that point, there could be a tunable for how much effort
to expend. Perhaps I'm willing to decompress an inline value,
but not retrieve an out-of-line one? For the TOAST compression
algorithm I'm not sure of the balance between compression
and decompression effort; I know gzip decompression is pretty cheap.

-Chap


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Re: [BUGS] [HACKERS] Re: Postgresql bug report - unexpected behavior of suppress_redundant_updates_trigger

[David G. Johnston]

On Mon, Jun 19, 2017 at 2:10 PM, Tom Lane  wrote:
> Alvaro Herrera  writes:
>> Tom Lane wrote:
>>> ... If the trigger is succeeding (ie,
>>> detecting a no-op update) often enough that it would be worth that,
>>> you've really got an application-stupidity problem to fix.
>
>> ISTM the whole point of suppress_redundant_updates_trigger is to cope
>> with application stupidity.
>
> I think it's a suitable band-aid for limited amounts of stupidity.
> But it eliminates only a small fraction of the total overhead involved
> in a useless update command.  So I remain of the opinion that if that's
> happening a lot, you're better off fixing the problem somewhere upstream.

At first glance I think I'd rather have it do the correct thing all of
the time, even if it takes longer, so that my only trade-off decision
is whether to improve performance by fixing the application.

Ideally if the input tuple wouldn't require compression we wouldn't
bother to decompress the stored tuple.

David J.


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Re: [BUGS] [HACKERS] Re: Postgresql bug report - unexpected behavior of suppress_redundant_updates_trigger

[Tom Lane]

Alvaro Herrera  writes:
> Tom Lane wrote:
>> ... If the trigger is succeeding (ie,
>> detecting a no-op update) often enough that it would be worth that,
>> you've really got an application-stupidity problem to fix.

> ISTM the whole point of suppress_redundant_updates_trigger is to cope
> with application stupidity.

I think it's a suitable band-aid for limited amounts of stupidity.
But it eliminates only a small fraction of the total overhead involved
in a useless update command.  So I remain of the opinion that if that's
happening a lot, you're better off fixing the problem somewhere upstream.

regards, tom lane


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Re: [BUGS] [HACKERS] Re: Postgresql bug report - unexpected behavior of suppress_redundant_updates_trigger

[Alvaro Herrera]

Tom Lane wrote:

> As I mentioned upthread, it'd certainly be possible for the trigger
> to iterate through the fields in a datatype-aware fashion and undo
> compression or out-of-lineing before the comparison.  But that would
> eat a lot more cycles than the current implementation, and it seems
> dubious that it's worth it.  If the trigger is succeeding (ie,
> detecting a no-op update) often enough that it would be worth that,
> you've really got an application-stupidity problem to fix.

ISTM the whole point of suppress_redundant_updates_trigger is to cope
with application stupidity.

-- 
Álvaro Herrerahttps://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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