Re: shared-memory based stats collector - v70

On Thu, 11 Aug 2022 at 02:11, Drouvot, Bertrand <bdrouvot(at)amazon(dot)com> wrote:
>
> As Andres was not -1 about that idea (as it should be low cost to add
> and maintain) as long as somebody cares enough to write something: then
> I'll give it a try and submit a patch for it.

I agree it would be a useful feature. I think there may be things to
talk about here though.

1) Are you planning to go through the local hash table and
LocalSnapshot and obey the consistency mode? I was thinking a flag
passed to build_snapshot to request global mode might be sufficient
instead of a completely separate function.

2) When I did the function attached above I tried to avoid returning
the whole set and make it possible to process them as they arrive. I
actually was hoping to get to the point where I could start shipping
out network data as they arrive and not even buffer up the response,
but I think I need to be careful about hash table locking then.

3) They key difference here is that we're returning whatever stats are
in the hash table rather than using the catalog to drive a list of id
numbers to look up. I guess the API should make it clear this is what
is being returned -- on that note I wonder if I've done something
wrong because I noted a few records with InvalidOid where I didn't
expect it.

4) I'm currently looping over the hash table returning the records all
intermixed. Some users will probably want to do things like "return
all Relation records for all databases" or "return all Index records
for database id xxx". So some form of filtering may be best or perhaps
a way to retrieve just the keys so they can then be looked up one by
one (through the local cache?).

5) On that note I'm not clear how the local cache will interact with
these cross-database lookups. That should probably be documented...

--
greg