pgsql: Use fast path in plpgsql's RETURN/RETURN NEXT in more cases.

Use fast path in plpgsql's RETURN/RETURN NEXT in more cases.

exec_stmt_return() and exec_stmt_return_next() have fast-path code for
handling a simple variable reference (i.e. "return var") without going
through the full expression evaluation machinery.  For some reason,
pl_gram.y was under the impression that this fast path only applied for
record/row variables; but in reality code for handling regular scalar
variables has been there all along.  Adjusting the logic to allow that
code to be used actually results in a net savings of code in pl_gram.y
(by eliminating some redundancy), and it buys a measurable though not
very impressive amount of speedup.

Noted while fooling with my expanded-array patch, wherein this makes a much
bigger difference because it enables returning an expanded array variable
without an extra flattening step.  But AFAICS this is a win regardless,
so commit it separately.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/9e3ad1aac52454569393a947c06be0d301749362

Modified Files
--------------
src/pl/plpgsql/src/pl_exec.c |   10 ++++++----
src/pl/plpgsql/src/pl_gram.y |   42 ++++++++++++++++++++++--------------------
2 files changed, 28 insertions(+), 24 deletions(-)