Re: How to hint 2 coulms IS NOT DISTINCT FROM each other

On Fri, Oct 28, 2016 at 10:29 AM, Kim Rose Carlsen <krc@hiper.dk> wrote:
> Hi
>
> I was wondering if there is a way to hint that two columns in two different
> tables IS NOT DISTINCT FROM each other. So that the optimizer may assume if
> table_a.key = 'test' THEN table_b.key = 'test' .
>
> The equals operator already does this but it does not handle NULLS very well
> (or not at all). And it seems IS NOT DISTINCT FROM is not indexable and
> doesn't establish the same inference rules as equals.

The whole idea behing Postgres' query planner is that you don't have
to use any hints. Late model versions of postgres handle nulls fine,
but nulls are never "equal" to anything else. I.e. where xxx is null
works with indexes. Where x=y does not, since null <> null.

Suggestion for getting help, put a large-ish aka production sized
amount of data into your db, run your queries with explain analyze and
feed them to https://explain.depesz.com/ and post the links here along
with the slow queries. A lot of times the fix is non-obvious if you're
coming from another db with a different set of troubleshooting skills
for slow queries.