Tobias Bussmann has discovered an oddity with prepared statements.
Parallel scan is used with prepared statements, but only if they have
been created with protocol V3 "Parse".
If a prepared statement has been prepared with the SQL statement PREPARE,
it will never use a parallel scan.
I guess that is an oversight in commit 57a6a72b, right?
PrepareQuery in commands/prepare.c should call CompleteCachedPlan
with cursor options CURSOR_OPT_PARALLEL_OK, just like
exec_prepare_message in tcop/postgres.c does.
The attached patch fixes the problem for me.
Yours,
Laurenz Albe
