Re: Removing unneeded self joins

On 2/7/21 01:56, Hywel Carver wrote:
> On Wed, Jun 30, 2021 at 12:21 PM Andrey Lepikhov
> <a(dot)lepikhov(at)postgrespro(dot)ru <mailto:a(dot)lepikhov(at)postgrespro(dot)ru>> wrote:
> 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?
>
>
> My suggestion was not to remove it, but to avoid adding it in the first
> place. When your optimisation has found a join on a group of columns
> under a uniqueness constraint, you would do something like this (forgive
> the pseudo-code)
>
> foreach(column, join_clause) {
>   if(column.nullable) { // This condition is what I'm suggesting is added
>     add_null_test(column, IS_NOT_NULL);
>   }
> }
>
> But it may be that that's not possible or practical at this point in the
> code.
I think, such option will require to implement a new machinery to prove
that arbitrary column couldn't produce NULL value.

--
regards,
Andrey Lepikhov
Postgres Professional