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>

2 files changed, 41 insertions, 0 deletions

diff --git a/src/test/regress/expected/rangefuncs.out b/src/test/regress/expected/rangefuncs.out
index 00ef4210549..f06cfa4b21d 100644
--- a/src/test/regress/expected/rangefuncs.out
+++ b/src/test/regress/expected/rangefuncs.out
@@ -2076,3 +2076,33 @@ select x from int8_tbl, extractq2_2_opt(int8_tbl) f(x);
  -4567890123456789
 (5 rows)
 
+-- check handling of nulls in SRF results (bug #7808)
+create type foo2 as (a integer, b text);
+select *, row_to_json(u) from unnest(array[(1,'foo')::foo2, null::foo2]) u;
+ a |  b  |     row_to_json     
+---+-----+---------------------
+ 1 | foo | {"a":1,"b":"foo"}
+   |     | {"a":null,"b":null}
+(2 rows)
+
+select *, row_to_json(u) from unnest(array[null::foo2, null::foo2]) u;
+ a | b |     row_to_json     
+---+---+---------------------
+   |   | {"a":null,"b":null}
+   |   | {"a":null,"b":null}
+(2 rows)
+
+select *, row_to_json(u) from unnest(array[null::foo2, (1,'foo')::foo2, null::foo2]) u;
+ a |  b  |     row_to_json     
+---+-----+---------------------
+   |     | {"a":null,"b":null}
+ 1 | foo | {"a":1,"b":"foo"}
+   |     | {"a":null,"b":null}
+(3 rows)
+
+select *, row_to_json(u) from unnest(array[]::foo2[]) u;
+ a | b | row_to_json 
+---+---+-------------
+(0 rows)
+
+drop type foo2;
diff --git a/src/test/regress/sql/rangefuncs.sql b/src/test/regress/sql/rangefuncs.sql
index 9484023f97b..c8edc553bc3 100644
--- a/src/test/regress/sql/rangefuncs.sql
+++ b/src/test/regress/sql/rangefuncs.sql
@@ -639,3 +639,14 @@ explain (verbose, costs off)
 select x from int8_tbl, extractq2_2_opt(int8_tbl) f(x);
 
 select x from int8_tbl, extractq2_2_opt(int8_tbl) f(x);
+
+-- check handling of nulls in SRF results (bug #7808)
+
+create type foo2 as (a integer, b text);
+
+select *, row_to_json(u) from unnest(array[(1,'foo')::foo2, null::foo2]) u;
+select *, row_to_json(u) from unnest(array[null::foo2, null::foo2]) u;
+select *, row_to_json(u) from unnest(array[null::foo2, (1,'foo')::foo2, null::foo2]) u;
+select *, row_to_json(u) from unnest(array[]::foo2[]) u;
+
+drop type foo2;