Re: Oddity in EXPLAIN for foreign/custom join pushdown plans

On 2016/07/27 13:09, Ashutosh Bapat wrote:
> The patch always prints ForeignJoin when scanrelid <= 0, which would be
> odd considering that FDWs can now push down post-join operations. We
> need to device a better way to convey post-join operations. May be
> something like
> Foreign Grouping, aggregation on ...
> Foreign Join on ...

Good point!

The point of the proposed patch is to print "Foreign Join"/"Custom
Join", to show, in every text/xml/json/yaml format, that there are
multiple target relations involved in that join and to print those
relations, following the "Foreign Join"/"Custom Join" words, which I
think would be more machine-readable.

> But then the question is a foreign scan node can be pushing down many
> operations together e.g. join, aggregation, sort OR join aggregation and
> windowing OR join and insert. How would we clearly convey this? May be
> we say
> Foreign Scan
> operations: join on ..., aggregation, ...
>
> That wouldn't be so great and might be clumsy for many operations. Any
> better idea?

How about: when doing post scan/join operations remotely, print such
additional operations in EXPLAIN the same way as in the "Relations" or
"Remote SQL" info printed by postgres_fdw. Maybe something like this:

Foreign Scan/Join on target relation(s)
Output: ...
Relations: ...
Additional Operations: grouping, aggregate, ...
Remote SQL: ...

That seems not that clumsy to me.

Best regards,
Etsuro Fujita