TRUNCATE on foreign tables

Hello,

We right now don't support TRUNCATE on foreign tables.
It may be a strange missing piece and restriction of operations.
For example, if a partitioned table contains some foreign tables in its leaf,
user cannot use TRUNCATE command to clean up the partitioned table.

Probably, API design is not complicated. We add a new callback for truncate
on the FdwRoutine, and ExecuteTruncateGuts() calls it if relation is foreign-
table. In case of postgres_fdw, it also issues "TRUNCATE" command on the
remote side in the transaction block [*1].

[*1] But I hope oracle_fdw does not follow this implementation as is. :-)

How about your thought?

I noticed this restriction when I'm working on Arrow_Fdw enhancement for
"writable" capability. Because Apache Arrow [*2] is a columnar file format,
it is not designed for UPDATE/DELETE, but capable to bulk-INSERT.
It is straightforward idea to support only INSERT, and clear data by TRUNCATE.

[*2] Apache Arrow - https://arrow.apache.org/docs/format/Columnar.html

Best regards,
--
HeteroDB, Inc / The PG-Strom Project
KaiGai Kohei <kaigai(at)heterodb(dot)com>