Re: Hybrid Hash/Nested Loop joins and caching results from subplans

On Wed, 26 May 2021 at 14:19, Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> wrote:
> I just checked the latest code, looks like we didn't improve this situation except
> that we introduced a GUC to control it. Am I missing something? I don't have a
> suggestion though.

Various extra caching was done to help speed it up. We now cache the
volatility of RestrictInfo and PathTarget.

I also added caching for the hash function in RestrictInfo so that we
could more quickly determine if we can Result Cache or not.

There's still a bit of caching left that I didn't do. This is around
lateral_vars. I've nowhere to cache the hash function since that's
just a list of vars. At the moment we need to check that each time we
consider a result cache path. LATERAL joins are a bit less common so
I didn't think that would be a huge issue. There's always
enable_resultcache = off for people who cannot tolerate the overhead.

Also, it's never going to be 100% as fast as it was. We're considering
another path that we didn't consider before.

Did you do some performance testing that caused you to bring this topic up?

David