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Re: join removal

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Greg Stark <stark(at)mit(dot)edu>, "<pgsql-hackers(at)postgresql(dot)org>" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: join removal
Date: 2010-03-28 11:16:09
Message-ID: 603c8f071003280416w38d4d44cgb7cb301ce7adef6@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sun, Mar 28, 2010 at 12:19 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Sat, Mar 27, 2010 at 4:11 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> I'm not seeing how that would occur or would matter, but the worst case
>>> answer is to restart the scan of the SpecialJoinInfos from scratch any
>>> time you succeed in doing a join removal.
>
>> Well, say you have something like
>
>> SELECT 1 FROM A LEFT JOIN (B LEFT JOIN C ON Pbc) ON Pab
>
>> I think that the SpecialJoinInfo structure for the join between B and
>> C will match the criteria I articulated upthread, but the one for the
>> join between A and {B C} will not.  If C had not been in the query
>> from the begining then we'd have had:
>
>> SELECT 1 FROM A LEFT JOIN B ON Pab
>
>> ...under which circumstances the SpecialJoinInfo would match the
>> aforementioned criteria.
>
> I experimented with this and found that you're correct: the tests on the
> different SpecialJoinInfos do interact, which I hadn't believed
> initially.  The reason for this is that when we find out we can remove a
> particular rel, we have to remove the bits for it in other relations'
> attr_needed bitmaps.  In the above example, we first discover we can
> remove C.  Whatever B vars were used in Pbc will have an attr_needed
> set of {B,C}, and that C bit will prevent us from deciding that B can
> be removed when we are examining the upper SpecialJoinInfo (which will
> not consider C to be part of either min_lefthand or min_righthand).
> So we have to remove the C bits when we remove C.
>
> Attached is an extremely quick-and-dirty, inadequately commented draft
> patch that does it along the lines you are suggesting.  This was just to
> see if I could get it to work at all; it's not meant for application in
> anything like its current state.  However, I feel a very strong
> temptation to finish it up and apply it before we enter beta.  As you
> noted, this way is a lot cheaper than the original coding, whether one
> focuses on the cost of failing cases or the cost when the optimization
> is successful.  And if we hold it off till 9.1, then any bug fixes that
> have to be made in the area later will need to be made against two
> significantly different implementations, which will be a real PITA.
>
> Things that would need to be cleaned up:
>
> * I left join_is_removable where it was, mainly so that it was easy to
> compare how much it changed for this usage (not a lot).  I'm not sure
> that joinpath.c is an appropriate place for it anymore, though I can't
> see any obviously better place either.  Any thoughts on that?

I dislike the idea of leaving it in joinpath.c.  I don't even think it
properly belongs in the path subdirectory since it no longer has
anything to do with paths.  Also worth thinking about where we would
put the logic I pontificated about here:

http://archives.postgresql.org/pgsql-hackers/2009-10/msg01012.php

> * The removed relation has to be taken out of the set of baserels
> somehow, else for example the Assert in make_one_rel will fail.
> The current hack is to change its reloptkind to RELOPT_OTHER_MEMBER_REL,
> which I think is a bit unclean.  We could try deleting it from the
> simple_rel_array altogether, but I'm worried that that could result in
> dangling-pointer failures, since we're probably not going to go to the
> trouble of removing every single reference to the rel from the planner
> data structures.  A possible compromise is to invent another reloptkind
> value that is only used for "dead" relations.

+1 for dead relation type.

> * It would be good to not count the removed relation in
> root->total_table_pages.  If we made either of the changes suggested
> above then we could move the calculation of total_table_pages down to
> after remove_useless_joins and ignore the removed relation(s)
> appropriately.  Otherwise I'm tempted to just subtract off the relation
> size from total_table_pages on-the-fly when we remove it.

Sounds good.

> * I'm not sure yet about the adjustment of PlaceHolder bitmaps --- we
> might need to break fix_placeholder_eval_levels into two steps to get
> it right.

Not familiar enough with that code to comment.

> * Still need to reverse out the now-dead code from the original patch,
> in particular the NoOpPath support.

Yeah.

> Thoughts?

I'm alarmed by your follow-on statement that the current code can't
handle the two-levels of removable join case.  Seems like it ought to
form {B C} as a path over {B} and then {A B C} as a path over {A}.
Given that it doesn't, we already have a fairly serious bug, so we've
either got to put more work into the old implementation or switch to
this new one - and I think at this point you and I are both fairly
convinced that this is a better way going forward.

...Robert

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Subject: Re: join removal
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