Doc: Further update documentation for asynchronous execution.
Add a note about asynchronous execution by postgres_fdw when applied to
Append nodes that contain synchronous subplan(s) as well. Follow-up for
commit
27e1f1456.
Andrey Lepikhov and Etsuro Fujita
Discussion: https://postgr.es/m/
58fa2aa5-07f5-80b5-59a1-
fec8a349fee7%40postgrespro.ru