pgsql: Allow functions that return sets of tuples to return simple NULL

Allow functions that return sets of tuples to return simple NULLs.

ExecMakeTableFunctionResult(), which is used in SELECT FROM function(...)
cases, formerly treated a simple NULL output from a function that both
returnsSet and returnsTuple as a violation of the SRF protocol.  What seems
better is to treat a NULL output as equivalent to ROW(NULL,NULL,...).
Without this, cases such as SELECT FROM unnest(...) on an array of
composite are vulnerable to unexpected and not-very-helpful failures.
Old code comments here suggested an alternative of just ignoring
simple-NULL outputs, but that doesn't seem very principled.

This change had been hung up for a long time due to uncertainty about
how much we wanted to buy into the equivalence of simple NULL and
ROW(NULL,NULL,...).  I think that's been mostly resolved by the discussion
around bug #14235, so let's go ahead and do it.

Per bug #7808 from Joe Van Dyk.  Although this is a pretty old report,
fixing it smells a bit more like a new feature than a bug fix, and the
lack of other similar complaints suggests that we shouldn't take much risk
of destabilization by back-patching.  (Maybe that could be revisited once
this patch has withstood some field usage.)

Andrew Gierth and Tom Lane

Report: <E1TurJE-0006Es-TK@wrigleys.postgresql.org>

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/d8411a6c8b6e5f74b362ef2496723f7f88593737

Modified Files
--------------
src/backend/executor/execQual.c          | 125 ++++++++++++++++---------------
src/test/regress/expected/rangefuncs.out |  30 ++++++++
src/test/regress/sql/rangefuncs.sql      |  11 +++
3 files changed, 106 insertions(+), 60 deletions(-)