Re: [HACKERS] postgres_fdw bug in 9.6

>
>> Hmm. If I understand the patch correctly, it does not return any path
>> when merge join is allowed and there are merge clauses but no hash
>> clauses. In this case we will not create a foreign join path, loosing
>> some optimization. If we remove GetExistingLocalJoinPath, which
>> returns a path in those cases as well, we have a regression in
>> performance.
>
>
> Ok, will revise, but as I mentioned upthread, I'm not sure it's a good idea
> to search the pathlist to get a merge join even in this case.  I'd vote for
> creating a merge join path from the inner/outer paths in this case as well.
> I think that would simplify the code as well.

Creating a new path 1. requires memory 2. spends CPU cycles in costing
and creating it 3. requires a search in inner and outer relations'
pathlists (see my earlier reply). Searching for an existing path just
requires a search in one relation's pathlist. The path should be
there. Why do we want to spend extra resources in creating a new path
when an old one exists and searching it is more efficient.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company