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Re: join removal

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
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 04:19:39
Message-ID: 11346.1269749979@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
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?

* 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.

* 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.

* 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.

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

Thoughts?

			regards, tom lane


Attachment: alternative-join-removal-0.patch
Description: text/x-patch (20.1 KB)

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