Re: Add semi-join pushdown to postgres_fdw

Hi, Alexander!

On Tue, Oct 31, 2023 at 1:07 PM Alexander Pyhalov
<a.pyhalov@postgrespro.ru> wrote:
> There are several cases when we can't push down semi-join in current
> patch.
>
> 1) When target list has attributes from inner relation, which are
> equivalent to some attributes of outer
> relation, we fail to notice this.
>
> 2) When we examine A join B and decide that we can't push it down, this
> decision is final - we state it in fdw_private of joinrel,
> and so if we consider joining these relations in another order, we don't
> reconsider.
> This means that if later examine B join A, we don't try to push it down.
> As semi-join can be executed as JOIN_UNIQUE_INNER or JOIN_UNIQUE_OUTER,
> this can be a problem - we look at some of these paths and remember that
> we can't push down such join.

Thank you for the revision.

I've revised the patch myself.  I've replaced StringInfo with
additional conds into a list of strings as I proposed before.  I think
the code became much clearer.  Also, it gets rid of some unnecessary
allocations.

I think the code itself is not in bad shape.  But patch lacks some
high-level description of semi-joins processing as well as comments on
each manipulation with additional conds.  Could you please add this?

------
Regards,
Alexander Korotkov