Re: Allow table AM's to cache stuff in relcache

On Fri, Jun 14, 2019 at 5:40 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
> > Index AM's can cache stuff in RelationData->rd_amcache. In the zedstore
> > table AM we've been hacking on, I'd like to also use rd_amcache to cache
> > some information, but that's not currently possible, because rd_amcache
> > can only be used by index AMs, not table AMs.
> > Attached patch allows rd_amcache to also be used by table AMs.
>
> Seems reasonable.

+1.

> > In the patch, I documented that rd_amcache must be allocated in
> > CacheMemoryContext, or in rd_indexcxt if it's an index. It works, but
> > it's a bit weird.
>
> Given the way the patch is implemented, it doesn't really matter which
> context it's in, does it? The retail pfree is inessential but also
> harmless, if rd_amcache is in rd_indexcxt. So we could take out the
> "must". I think it's slightly preferable to use rd_indexcxt if available,
> to reduce the amount of loose junk in CacheMemoryContext.

I agree that for indexes the context used won't make much difference.
But IMHO avoiding some bloat in CacheMemoryContext is a good enough
reason to document using rd_indexcxt when available.

> > It would nice to have one memory context in every
> > relcache entry, to hold all the stuff related to it, including
> > rd_amcache. In other words, it would be nice if we had "rd_indexcxt" for
> > tables, too, not just indexes. That would allow tracking memory usage
> > more accurately, if you're debugging an out of memory situation for example.
>
> We had some discussion related to that in the "hyrax
> vs. RelationBuildPartitionDesc" thread. I'm not quite sure where
> we'll settle on that, but some redesign seems inevitable.

There wasn't any progress on this since last month, and this patch
won't make the situation any worse. I'll mark this patch as ready for
committer, as it may save some time for people working on custom table
AM.