Re: [PoC] Reducing planning time when tables have many partitions

Hello David,

I really appreciate your quick reply.

On Wed, Aug 9, 2023 at 7:28 PM David Rowley <dgrowleyml@gmail.com> wrote:
> If 0004 is adding an em_index to mark the index into
> PlannerInfo->eq_members, can't you use that in
> setup_eclass_member[_strict]_iterator to loop to verify that the two
> methods yield the same result?
>
> i.e:
>
> + Bitmapset *matching_ems = NULL;
> + memcpy(&idx_iter, iter, sizeof(EquivalenceMemberIterator));
> + memcpy(&noidx_iter, iter, sizeof(EquivalenceMemberIterator));
> +
> + idx_iter.use_index = true;
> + noidx_iter.use_index = false;
> +
> + while ((em = eclass_member_iterator_strict_next(&noidx_iter)) != NULL)
> +     matching_ems = bms_add_member(matching_ems, em->em_index);
> +
> + Assert(bms_equal(matching_ems, iter->matching_ems));
>
> That should void the complaint that the Assert checking is too slow.
> You can also delete the list_ptr_cmp function too (also noticed a
> complaint about that).

Thanks for sharing your idea regarding this verification. It looks
good to solve the degradation problem in assert-enabled builds. I will
try it.

> For the 0003 patch.  Can you explain why you think these fields should
> be in RangeTblEntry rather than RelOptInfo? I can only guess you might
> have done this for memory usage so that we don't have to carry those
> fields for join rels?  I think RelOptInfo is the correct place to
> store fields that are only used in the planner.  If you put them in
> RangeTblEntry they'll end up in pg_rewrite and be stored for all
> views.  Seems very space inefficient and scary as it limits the scope
> for fixing bugs in back branches due to RangeTblEntries being
> serialized into the catalogues in various places.

This change was not made for performance reasons but to avoid null
reference exceptions. The details are explained in my email [1]. In
brief, the earlier patch did not work because simple_rel_array[i]
could be NULL for some 'i', and we referenced such a RelOptInfo. For
example, the following code snippet in add_eq_member() does not work.
I inserted "Assert(rel != NULL)" into this code, and then the
assertion failed. So, I moved the indexes to RangeTblEntry to address
this issue, but I don't know if this solution is good. We may have to
solve this in a different way.

=====
@@ -572,9 +662,31 @@ add_eq_member(EquivalenceClass *ec, Expr *expr,
Relids relids,
+    i = -1;
+    while ((i = bms_next_member(expr_relids, i)) >= 0)
+    {
+        RelOptInfo *rel = root->simple_rel_array[i];
+
+        rel->eclass_member_indexes =
bms_add_member(rel->eclass_member_indexes, em_index);
+    }
=====

On Wed, Aug 9, 2023 at 8:54 PM David Rowley <dgrowleyml@gmail.com> wrote:
> So, I have three concerns with this patch.

> I think the best way to move this forward is to explore not putting
> partitioned table partitions in EMs and instead see if we can
> translate to top-level parent before lookups.  This might just be too
> complex to translate the Exprs all the time and it may add overhead
> unless we can quickly determine somehow that we don't need to attempt
> to translate the Expr when the given Expr is already from the
> top-level parent. If that can't be made to work, then maybe that shows
> the current patch has merit.

I really appreciate your detailed advice. I am sorry that I will not
be able to respond for a week or two due to my vacation, but I will
explore and work on these issues. Thanks again for your kind reply.

[1] https://www.postgresql.org/message-id/CAJ2pMkYR_X-%3Dpq%2B39-W5kc0OG7q9u5YUwDBCHnkPur17DXnxuQ%40mail.gmail.com

-- 
Best regards,
Yuya Watari