On Sat, 9 Mar 2019 at 00:30, Artur Zając <azajac@ang.com.pl> wrote:
> Is there any reason that „NOT DISTINCT FROM” can’t be autotransformed to „=” when value on right side of expression
isnot NULL or is this any way to use index with „IS NOT DISTINCT FROM” statement?
Probably nothing other than nobody has done it yet. It might be
reasonable to have some transformation stage called from
distribute_restrictinfo_to_rels() when adding single rel RestrictInfos
to RTE_RELATION base rels. It's only these you can check for NOT NULL
constraints, i.e. not so possible with rtekinds such as RTE_FUNCTION
and the like.
It becomes more complex if you consider that someone might have added
a partial index on the relation that matches the IS NOT DISTINCT FROM
clause. In this case, they might not be happy that their index can no
longer be used. Fixing that would require some careful surgery on
predicate_implied_by() to teach it about IS NOT DISTINCT FROM clauses.
However, that seems to go a step beyond what predicate_implied_by()
does for now. Currently, it only gets to know about quals. Not the
relations they belong to, so there'd be no way to know that the NOT
NULL constraint exists from there. I'm not sure if there's a good
reason for this or not, it might be because it's not been required
before. It gets more complex still if you want to consider other
quals in the list to prove not nullness.
In short, probably possible, but why not just write an equality
clause, if you know NULLs are not possible?
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
