Re: Removing unneeded self joins

On Mon, Jul 5, 2021 at 2:20 PM Andrey Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru>
wrote:

> 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.
>

Got it, and it makes sense to me that this would be out of scope for this
change.

I remember in the previous conversation about this, Tomas acknowledged that
while there are some silly queries that would benefit from this change,
there are also some well-written ones (e.g. properly denormalised table
structures, with decomposed views that need joining together in some
queries). So the optimization needs to be essentially free to run to
minimise impact on other queries.

Looking through the email chain, a previous version of this patch added
~0.6% to planning time in the worst case tested - does that meet the
"essentially free" requirement?