Restore the ability to run pl/pgsql expression queries in parallel.HEADmaster

pl/pgsql's notion of an "expression" is very broad, encompassing
any SQL SELECT query that returns a single column and no more than
one row.  So there are cases, for example evaluation of an aggregate
function, where the query involves significant work and it'd be useful
to run it with parallel workers.  This used to be possible, but
commits 3eea7a0c9 et al unintentionally disabled it.

The simplest fix is to make exec_eval_expr() pass maxtuples = 0
rather than 2 to exec_run_select().  This avoids the new rule that
we will never use parallelism when a nonzero "count" limit is passed
to ExecutorRun().  (Note that the pre-3eea7a0c9 behavior was indeed
unsafe, so reverting that rule is not in the cards.)  The reason
for passing 2 before was that exec_eval_expr() will throw an error
if it gets more than one returned row, so we figured that as soon
as we have two rows we know that will happen and we might as well
stop running the query.  That choice was cost-free when it was made;
but disabling parallelism is far from cost-free, so now passing 2
amounts to optimizing a failure case at the expense of useful cases.
An expression query that can return more than one row is certainly
broken.  People might now need to wait a bit longer to discover such
breakage; but hopefully few will use enormously expensive cases as
their first test of new pl/pgsql logic.

Author: Dipesh Dhameliya <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/CABgZEgdfbnq9t6xXJnmXbChNTcWFjeM_6nuig41tm327gYi2ig@mail.gmail.com
Backpatch-through: 13

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