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

On 2016/07/29 13:28, Ashutosh Bapat wrote:

I wrote:
>             Probably something like this:
>
>                Foreign Processing
>                  Remote Operations: ...
>
>             In the Remote Operations line, the FDW/extension could print
>             any info
>             about remote operations, eg, "Scan/Join + Aggregate".

I wrote:
>     I intentionally chose that word and thought we could leave detailed
>     descriptions about remote operations to the FDW/extension; a broader
>     word like "Processing" seems to work well because we allow various
>     kinds of operations to the remote side, in addition to scans/joins,
>     to be performed in that one Foreign Scan node indicated by "Foreign
>     Processing", such as aggregation, window functions, distinct, order
>     by, row locking, table modification, or combinations of them.

> "Scan" is a better word than "Processing". From plan's perspective it's
> ultimately a Scan (on the data produced by the foreign server) and not
> processing.

Exactly, but one thing I'm concerned about is the table modification 
case; the EXPLAIN output would be something like this:
  Foreign Scan    Remote SQL: INSERT INTO remote_table VALUES ...

That would be strange, so I think a more broader word is better.  I 
don't think "Foreign Processing" is best.  "Foreign Upper" might be much 
better because the corresponding path is created by calling 
GetForeignUpperPaths.

Also for a Foreign Scan representing a foreign join, I think "Foreign 
Join" is better than "Foreign Scan".  Here is an example:
  Foreign Join on foreign_table1, foreign_table2    Remote SQL: SELECT ... FROM remote_table1 INNER JOIN remote_table2

WHERE ...

I think "Foreign Join" better indicates that foreign tables listed after 
that (ie, foreign_table1 and foreign_table2 in the example) are joining 
(or component) tables of this join, than "Foreign Scan".

Best regards,
Etsuro Fujita