Re: WIP: Aggregation push-down

Antonin Houska <ah(at)cybertec(dot)at> wrote:

> This is another version of the patch.

> shard.tgz demonstrates the typical postgres_fdw use case. One query shows base
> scans of base relation's partitions being pushed to shard nodes, the other
> pushes down a join and performs aggregation of the join result on the remote
> node. Of course, the query can only references one particular partition, until
> the "partition-wise join" [1] patch gets committed and merged with this my
> patch.

Since [1] is already there, the new version of shard.tgz shows what I consider
the typical use case. (I'm aware of the postgres_fdw regression test failures,
I'll try to fix them all in the next version.)

Besides that:

* A concept of "path unique keys" has been introduced. It helps to find out if
the final relation appears to generate a distinct set of grouping keys. If
that happens, the final aggregation is replaced by mere call of aggfinalfn()
function on each transient state value.

* FDW can sort rows by aggregate.

* enable_agg_pushdown GUC was added. The default value is false.

* I fixed errors reported during the previous CF.

* Added a few more regression tests.

I'm not about to add any other features now. Implementation of the missing
parts (see the TODO comments in the code) is the next step. But what I'd
appreciate most is a feedback on the design. Thanks.

> [1] https://commitfest.postgresql.org/14/994/

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at