Defer selection of asynchronous subplans until the executor initialization stage

On 7/5/21 21:05, Etsuro Fujita wrote:
> I think it would be better to start a new thread for this, and add the
> patch to the next CF so that it doesn’t get lost.

Current implementation of async append choose asynchronous subplans at
the phase of an append plan creation. This is safe approach, but we
loose some optimizations, such of flattening trivial subqueries and
can't execute some simple queries asynchronously. For example:

EXPLAIN (ANALYZE, TIMING OFF, SUMMARY OFF, COSTS OFF)
(SELECT * FROM f1 WHERE a < 10) UNION ALL
(SELECT * FROM f2 WHERE a < 10);

But, as I could understand, we can choose these subplans later, at the
init append phase when all optimizations already passed.
In attachment - implementation of the proposed approach.

Initial script for the example see in the parent thread [1].

[1]
https://www.postgresql.org/message-id/a38bb206-8340-9528-5ef6-37de2d5cb1a3%40postgrespro.ru

--
regards,
Andrey Lepikhov
Postgres Professional