Re: Combining Aggregates

Robert Haas wrote:

> Now, let's suppose that the user sets up a sharded table and then
> says: SELECT a, SUM(b), AVG(c) FROM sometab.  At this point, what we'd
> like to have happen is that for each child foreign table, we go and
> fetch partially aggregated results.  Those children might be running
> any version of PostgreSQL - I was not assuming that we'd insist on
> matching major versions, although of course that could be done - and
> there would probably need to be a minimum version of PostgreSQL
> anyway.  They could even be running some other database.  As long as
> they can spit out partial aggregates in a format that we can
> understand, we can deserialize those aggregates and run combine
> functions on them.  But if the remote side is, say, MariaDB, it's
> probably much easier to get it to spit out something that looks like a
> PostgreSQL array than it is to make it spit out some bytea blob that's
> in an entirely PostgreSQL-specific format.

Basing parts of the Postgres sharding mechanism on FDWs sounds
acceptable.  Trying to design things so that *any* FDW can be part of a
shard, so that you have some shards in Postgres and other shards in
MariaDB, seems ludicrous to me.  Down that path lies madness.

In fact, trying to ensure cross-major-version compatibility already
sounds like asking for too much.  Requiring matching major versions
sounds a perfectly acceptable restricting to me.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services