On Sat, 9 Mar 2019 at 00:30, Artur Zając <aza...@ang.com.pl> wrote: > Is there any reason that „NOT DISTINCT FROM” can’t be autotransformed to „=” > when value on right side of expression is not 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
