Subject: Re: Removing unneeded self joins
To: Hywel Carver <hywel@skillerwhale.com>, pgsql-hackers@lists.postgresql.org
Cc: Konstantin Knizhnik <knizhnik@garret.ru>
References: <64486b0b-0404-e39e-322d-0801154901f3@postgrespro.ru>
 <21e15759-dba5-2d2b-dd03-06943ad40378@postgrespro.ru>
 <161553990603.28624.7702383984090736980.pgcf@coridan.postgresql.org>
From: Andrey Lepikhov <a.lepikhov@postgrespro.ru>
Organization: Postgres Professional
Message-ID: <2a6e540a-4c9f-9402-2e02-71e35fcd4dd1@postgrespro.ru>
Date: Wed, 30 Jun 2021 14:21:15 +0300
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On 12/3/21 12:05, Hywel Carver wrote:
> I've built and tested this, and it seems to function correctly to me. One question I have is whether the added "IS NOT NULL" filters can be omitted when they're unnecessary. Some of the resulting plans included an "IS NOT NULL" filter on a non-nullable column. To be clear, this is still an improvement (to me) without that.
I think, here we could ask more general question: do we want to remove a 
'IS NOT NULL' clause from the clause list if the rest of the list 
implicitly implies it?

Right now we don't analyze list of clauses at all:
CREATE TABLE a (x int);

EXPLAIN (COSTS OFF) 
                                  SELECT * FROM a WHERE (x < 1) AND (X 
IS NOT NULL) AND
   (x < 1) AND (X IS NOT NULL);
                                QUERY PLAN
-------------------------------------------------------------------------
  Seq Scan on a
    Filter: ((x IS NOT NULL) AND (x IS NOT NULL) AND (x < 1) AND (x < 1))

And even worse:
EXPLAIN (ANALYZE, VERBOSE) 
                                                                SELECT * 
FROM a WHERE (X IS NOT NULL) AND (X IS NULL);
                                              QUERY PLAN 

----------------------------------------------------------------------------------------------------
  Seq Scan on public.a  (cost=0.00..15.00 rows=87 width=4) (actual 
time=0.136..0.136 rows=0 loops=1)
    Output: x
    Filter: ((a.x IS NOT NULL) AND (a.x IS NULL))
    Rows Removed by Filter: 1000

It could reduce a number of selectivity mistakes, but increase CPU 
consumption.
If we had such a clause analyzing machinery, we could trivially remove 
this unneeded qual.

-- 
regards,
Andrey Lepikhov
Postgres Professional