Re: Removing unneeded self joins

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