Re: apply_scanjoin_target_to_paths and partitionwise join

On 24/7/2024 15:22, Ashutosh Bapat wrote:
> On Wed, Jul 24, 2024 at 9:42 AM Richard Guo <guofenglinux@gmail.com> wrote:
>> Is there a specific query that demonstrates benefits from this change?
>> I'm curious about scenarios where a partitionwise join runs slower
>> than a non-partitionwise join.
> 
> [1] provides a testcase where a nonpartitionwise join is better than
> partitionwise join. This testcase is derived from a bug reported by an
> EDB customer. [2] is another bug report on psql-bugs.
I haven't passed through the patch yet, but can this issue affect the 
decision on what to push down to foreign servers: a whole join or just a 
scan of two partitions?
If the patch is related to the pushdown decision, I'd say it is quite an 
annoying problem for me. From time to time, I see cases where JOIN 
produces more tuples than both partitions have in total - in this case, 
it would be better to transfer tables' tuples to the main instance 
before joining them.

-- 
regards, Andrei Lepikhov