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

Hello,

I really appreciate sharing very useful scripts and benchmarking results.

On Fri, Jul 28, 2023 at 6:51 PM Ashutosh Bapat
<ashutosh.bapat.oss@gmail.com> wrote:
> Given that most of the developers run assert enabled builds it would
> be good to bring down the degradation there while keeping the
> excellent speedup in non-assert builds.

From my observation, this degradation in assert enabled build is
caused by verifying the iteration results of EquivalenceMembers. My
patch uses Bitmapset-based indexes to speed up the iteration. When
assertions are enabled, we verify that the result of the iteration is
the same with and without the indexes. This verification results in
executing a similar loop three times, which causes the degradation. I
measured planning time by using your script without this verification.
The results are as follows:

Master: 144.55 ms
Patched (v19): 529.85 ms
Patched (v19) without verification: 78.84 ms
(*) All runs are with assertions.

As seen from the above, verifying iteration results was the cause of
the performance degradation. I agree that we should avoid such
degradation because it negatively affects the development of
PostgreSQL. Removing the verification when committing this patch is
one possible option.

> Queries on partitioned tables eat a lot of memory anyways, increasing
> that further should be avoided.
>
> I have not studied the patches. But I think the memory increase has to
> do with our Bitmapset structure. It's space inefficient when there are
> thousands of partitions involved. See my comment at [2]

Thank you for pointing this out. I have never considered the memory
usage impact of this patch. As you say, the Bitmapset structure caused
this increase. I will try to look into this further.