[BUGS] BUG #14676: neqsel is NULL dumb

The following bug has been logged on the website:

Bug reference:      14676
Logged by:          Marko Tiikkaja
Email address:      marko@joh.to
PostgreSQL version: 9.6.3
Operating system:   Linux
Description:

I'm having an issue with a case where a column is mostly NULLs and I'm doing
an inequality query on the column:

=# create table foo(nullable int);
CREATE TABLE

=# insert into foo select case when i = 1 then i else null end from
generate_series(1, 1000) gs(i);
INSERT 0 1000

=# analyze foo;
ANALYZE

=# explain select * from foo where nullable <> 1;                     QUERY PLAN
------------------------------------------------------Seq Scan on foo  (cost=0.00..16.50 rows=999 width=4)  Filter:
(nullable<> 1)
 
(2 rows)

This seems to be because neqsel() doesn't take at all into account that both
operators will exclude NULL rows, and does a simple  1.0 - eqsel().  This
also means that a partial index such as:
  create index on foo(othercolumn) where nullable <> 1

will never be used.


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