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Re: OUTER JOIN performance regression remains in 8.3beta4

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: pgsql-hackers(at)postgresql(dot)org, pgsql-patches(at)postgresql(dot)org
Subject: Re: OUTER JOIN performance regression remains in 8.3beta4
Date: 2008-01-08 21:11:16
Message-ID: (view raw or whole thread)
Lists: pgsql-hackerspgsql-patches
I wrote:
> Haven't looked closely at how to fix 8.2, yet.

After some study it seems that the simplest, most reliable fix for 8.2
is to dike out the code that removes "redundant" outer join conditions
after propagating a constant across them.  This gives the right answer
in the cases of concern (where we actually need the join condition) and
doesn't really add much overhead in the cases where we don't need it.

One small problem is that the join condition is redundant with the
generated constant-equality constraints (mostly so, even if not entirely
so) which will cause the planner to underestimate the size of the join,
since clausesel.c is not very bright at all about redundant conditions.
However, we already have a hack we can use for that: we can force the
cached selectivity estimate for the join clause to 1.0, so that it's
not considered to reduce the join size any more than the constant
conditions already did.  (This is also a problem in my earlier patch
for 8.3, with the same fix possible.)

That leads to the attached very simple patch.  There is some dead code
left behind, but it doesn't seem worth removing it.

I'm rather tempted to patch 8.1 similarly, even though it doesn't fail
on the known test case --- I'm far from convinced that there are no
related cases that will make it fail, and in any case it's getting the
selectivity wrong.  8.0 and before don't try to propagate constants
like this, so they're not at risk.

Comparing the behavior of this to my patch for HEAD, I am coming to the
conclusion that this is actually a *better* planning method than
removing the redundant join conditions, even when they're truly
rendundant!  The reason emerges as soon as you look at cases involving
more than a single join.  If we strip the join condition from just one
of the joins, then we find that the planner insists on doing that join
last, whether it's a good idea or not, because clauseful joins are
always preferred to clauseless joins in the join search logic.  What's
worse, knowing that this is an outer join, is that the only available
plan type for a clauseless outer join is a NestLoop with the inner side
on the right, which again may be a highly nonoptimal way to do it.

None of this matters a whole lot if the pushed-down constant conditions
select single rows, but it does if they select multiple rows.  I'm
trying this in the regression database:

select * from tenk1 a left join tenk1 b on (a.hundred = b.hundred)
  left join tenk1 c on (b.hundred = c.hundred) where a.hundred = 42;

and finding patched 8.2 about 2X faster than 8.3 because it selects a
better plan that avoids multiple rescans of subplans.

So I'm coming around to the idea that getting rid of the "redundant"
join conditions is foolish micro-optimization, and we should leave
them in place even when we know they're redundant.  The extra execution
cycles paid to test the condition don't amount to much in any case,
and the risk of getting a bad plan is too high.

This is a reasonably simple adjustment to my prior patch for 8.3,
which I will go ahead and make if there are no objections...

			regards, tom lane

Attachment: const-propagation-8.2.patch
Description: application/octet-stream (8.5 KB)

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