Re: BUG #17844: Memory consumption for memoize node

> Another thing that came to mind is that we don't track the memory for
> the cache key. So that could account for some additional memory usage
> with Memoize. I have a patch locally to fix that. Likely that would be
> a master-only fix, however. I doubt that's accounting for much of the
> extra memory you're reporting anyway. In hindsight, we should be
> tracking that, but I think at the time I was writing this code, I had
> thoughts that it wasn't much memory compared to storing the cached
> tuples. I now think differently.

I've also attached the have_memoize_track_cachekey_memory.patch to
address this.  I intend this one for master only.  I considered if
maybe the executor changes without the planner changes could be
backpatched, but I don't think that's a good idea. It wouldn't cause
plan stability problems, but it could cause executor performance
changes if we start evicting more cache entries due to memory
pressure.

  mstate->mem_used -= EMPTY_ENTRY_MEMORY_BYTES(entry);
+ mstate->mem_used -= sizeof(MemoizeKey) + GetMemoryChunkSpace(key->params);

It seems that the memory used by key is already accounted for in
EMPTY_ENTRY_MEMORY_BYTES.  I wonder if this change is needed.

Also I'm kinda confused about using MinimalTuple->t_len vs. using
GetMemoryChunkSpace(MinimalTuple).  Why do we choose t_len rather than
GetMemoryChunkSpace in EMPTY_ENTRY_MEMORY_BYTES and CACHE_TUPLE_BYTES?

Thanks