Re: [PERFORM] Hints proposal

От: Jeff Davis
Тема: Re: [PERFORM] Hints proposal
Дата: ,
Msg-id: 1160757954.31966.215.camel@dogma.v10.wvs
(см: обсуждение, исходный текст)
Ответ на: Re: [PERFORM] Hints proposal  (Christopher Browne)
Список: pgsql-hackers

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Re: [PERFORM] Hints proposal  (Tom Lane, )
 Re: [PERFORM] Hints proposal  (Jeff Davis, )
 Re: [PERFORM] Hints proposal  (Christopher Browne, )
  Re: [PERFORM] Hints proposal  (Jeff Davis, )

On Thu, 2006-10-12 at 23:12 -0400, Christopher Browne wrote:
> > No, I don't have any idea, except that it would be less push-back
> > than changing a language that's embedded in client code. Also, I see
> > no reason to think that a hint would not be obsolete upon a new
> > release anyway.
> I see *plenty* of reason.
> 1.  Suppose the scenario where Hint h was useful hasn't been affected
>     by *any* changes in how the query planner works in the new
>     version, it *obviously* continues to be necessary.
> 2.  If Version n+0.1 hasn't resolved all/most cases where Hint h was
>     useful in Version n, then people will entirely reasonably expect
>     for Hint h to continue to be in effect in version n+0.1

Fair enough. I had considered those situations, but a lot of people are
talking about "I need a better plan now, can't wait for planner
improvements". Also, even if the hint is still useful, I would think
that on a new version you'd want to test to see how useful it still is.

> 3.  Suppose support for Hint h is introduced in PostgreSQL version
>     n, and an optimization that makes it obsolete does not arrive
>     until version n+0.3, which is quite possible.  That hint has been
>     carried forward for 2 versions already, long enough for client
>     code that contains it to start to ossify.  (After all, if
>     developers get promoted to new projects every couple of years,
>     two versions is plenty of time for the original programmer to 
>     be gone...)

Ok, that is a good reason. But it's not helped at all by putting the
hints in the queries themselves.

> > "Little better" is all I was going for. I was just making the
> > observation that we can separate two concepts:
> > (1) Embedding code in the client's queries, which I see as very
> > undesirable and unnecessary
> > (2) Providing very specific hints
> >
> > which at least gives us a place to talk about the debate more
> > reasonably.
> It seems to me that there is a *LOT* of merit in trying to find
> alternatives to embedding code into client queries, to be sure.

I think almost any alternative to client query hints is worth

> >> The right way to think about it is to ask why is the planner not
> >> picking the right plan to start with --- is it missing a
> >> statistical correlation, or are its cost parameters wrong for a
> >> specific case, or is it perhaps unable to generate the desired plan
> >> at all?  (If the latter, no amount of hinting is going to help.)
> >> If it's a statistics or costing problem, I think the right thing is
> >> to try to fix it with hints at that level.  You're much more likely
> >> to fix the behavior across a class of queries than you will be with
> >> a hint textually matched to a specific query.
> >
> > Agreed.
> That's definitely a useful way to look at the issue, which seems to be
> lacking in many of the cries for hints.
> Perhaps I'm being unfair, but it often seems that people demanding
> hinting systems are uninterested in why the planner is getting things
> wrong.  Yes, they have an immediate problem (namely the wrong plan
> that is getting generated) that they want to resolve.
> But I'm not sure that you can get anything out of hinting without
> coming close to answering "why the planner got it wrong."

Right. And it's not always easy to determine why the planner got it
wrong without making it execute other plans through hinting :)

Note: I'll restate this just to be clear. I'm not advocating an overly-
specific, band-aid style hinting language. My only real concern is that
if one appears, I would not like it to appear in the client's queries. 

Same goes for more general kinds of hints. We don't want a bunch of
client queries to contain comments like "table foo has a
random_page_cost of 1.1". That belongs in the system catalogs.

Regards,Jeff Davis

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