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Third thoughts about the DISTINCT MAX() problem

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: Third thoughts about the DISTINCT MAX() problem
Date: 2008-03-28 21:51:43
Message-ID: 15346.1206741103@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
I just realized that the patch I applied here
http://archives.postgresql.org/pgsql-committers/2008-03/msg00531.php
for Taiki Yamaguchi's bug report here
http://archives.postgresql.org/pgsql-bugs/2008-03/msg00275.php
really doesn't work.  It assumes that an ungrouped aggregate
query can't return more than one row, which is true in straight
SQL ... but it's not true if you consider SRFs in the target list.
CVS HEAD gives the wrong answer for this example in the regression
database:

regression=# select max(unique1), generate_series(1,3) as g from tenk1 order by g desc;
 max  | g 
------+---
 9999 | 1
 9999 | 2
 9999 | 3
(3 rows)

because it wrongly supposes it can discard the ORDER BY.

So, back to the drawing board.  I had thought of two approaches to
fixing the problem instead of just dodging it.  Plan A was to
apply planagg.c's Aggref->Param substitution inside EquivalenceClasses,
as in the draft patch here:
http://archives.postgresql.org/pgsql-patches/2008-03/msg00388.php
which I didn't entirely like for reasons mentioned in that post.
Plan B was to try to revert to the way sort clause matching was
done pre-8.3, that is have make_sort_from_pathkeys check first
for a matching ressortgroupref tag before it goes looking for equal()
expressions.  I had actually tried to do that first but got hung
up on the problem of identifying the correct sort operator ---
just taking the exposed type of the targetlist entry doesn't always
work, because of binary-compatible cases (eg, tlist entry may say
it yields varchar but we need to find the text opclass).  Perhaps
thinking a bit harder would yield a solution though.

Does anyone have comments for or against either of these approaches,
or perhaps a Plan C to consider?

			regards, tom lane

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Subject: Re: Third thoughts about the DISTINCT MAX() problem
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