Re: Partition prune with stable Expr

On Mon, Sep 28, 2020 at 7:15 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> writes:
> > On Mon, Sep 28, 2020 at 4:46 AM David Rowley <dgrowleyml(at)gmail(dot)com>
> wrote:
> >> Thanks for showing an interest in partition pruning. Unfortunately,
> >> it's not possible to use stable functions to prune partitions during
> >> planning.
>
> > Sigh.. I understand you now, I ignored the plan can be cached for later
> use.
> > Without that, we should be able to prune with stable function.
>
> No, that's still wrong. The contract for a stable function is that
> its result won't change over execution of a single query; but that
> says *execution*, not *planning and execution*.
>

I have a slightly different opinion about the impact of "cached the plan
for later use will be wrong" now. Generic plan will never be partition
pruned plan since we don't know which partition to prune at plan time.
So for any cached plan, it is not a plan time partition pruned plan.
Partition prune with stable expr is still unacceptable even this is not
an issue but hope the snapshot issue will be the only one issue to
fix in future for this direction. I'd like to know if I am wrong again.

--
Best Regards
Andy Fan