Re: Problem, partition pruning for prepared statement with IS NULL clause.

On Sat, 7 Oct 2023 at 03:11, Sergei Glukhov <s.glukhov@postgrespro.ru> wrote:
> I noticed that combination of prepared statement with generic plan and
> 'IS NULL' clause could lead partition pruning to crash.

> Test case:
> ------
> set plan_cache_mode to force_generic_plan;
> prepare stmt AS select * from hp where a is null and b = $1;
> explain execute stmt('xxx');

Thanks for the detailed report and proposed patch.

I think your proposed fix isn't quite correct.  I think the problem
lies in InitPartitionPruneContext() where we assume that the list
positions of step->exprs are in sync with the keyno.  If you look at
perform_pruning_base_step() the code there makes a special effort to
skip over any keyno when a bit is set in opstep->nullkeys.

It seems that your patch is adjusting the keyno that's given to the
PruneCxtStateIdx() and it looks like (for your test case) it'll end up
passing keyno==0 when it should be passing keyno==1.  keyno is the
index of the partition key, so you can't pass 0 when it's for key
index 1.

I wonder if it's worth expanding the tests further to cover more of
the pruning cases to cover run-time pruning too.

David