Fix "cannot accept a set" error when only some arms of a CASE return a set.

In commit c1352052ef1d4eeb2eb1d822a207ddc2d106cb13, I implemented an
optimization that assumed that a function's argument expressions would
either always return a set (ie multiple rows), or always not.  This is
wrong however: we allow CASE expressions in which some arms return a set
of some type and others just return a scalar of that type.  There may be
other examples as well.  To fix, replace the run-time test of whether an
argument returned a set with a static precheck (expression_returns_set).
This adds a little bit of query startup overhead, but it seems barely
measurable.

Per bug #8228 from David Johnston.  This has been broken since 8.0,
so patch all supported branches.

src/backend/executor/execQual.c

@@ -1634,9 +1634,7 @@ tupledesc_match(TupleDesc dst_tupdesc, TupleDesc src_tupdesc)
  * init_fcache is presumed already run on the FuncExprState.
  *
  * This function handles the most general case, wherein the function or
- * one of its arguments might (or might not) return a set.	If we find
+ * one of its arguments can return a set.
- * no sets involved, we will change the FuncExprState's function pointer
- * to use a simpler method on subsequent calls.
  */
 static Datum
 ExecMakeFunctionResult(FuncExprState *fcache,
@@ -1906,13 +1904,12 @@ restart:
 		/*
 		 * Non-set case: much easier.
 		 *
-		 * We change the ExprState function pointer to use the simpler
+		 * In common cases, this code path is unreachable because we'd have
-		 * ExecMakeFunctionResultNoSets on subsequent calls.  This amounts to
+		 * selected ExecMakeFunctionResultNoSets instead.  However, it's
-		 * assuming that no argument can return a set if it didn't do so the
+		 * possible to get here if an argument sometimes produces set results
-		 * first time.
+		 * and sometimes scalar results.  For example, a CASE expression might
+		 * call a set-returning function in only some of its arms.
 		 */
-		fcache->xprstate.evalfunc = (ExprStateEvalFunc) ExecMakeFunctionResultNoSets;
-
 		if (isDone)
 			*isDone = ExprSingleResult;
 
@@ -2371,10 +2368,22 @@ ExecEvalFunc(FuncExprState *fcache,
 	init_fcache(func->funcid, func->inputcollid, fcache,
 				econtext->ecxt_per_query_memory, true);
 
-	/* Go directly to ExecMakeFunctionResult on subsequent uses */
+	/*
+	 * We need to invoke ExecMakeFunctionResult if either the function itself
+	 * or any of its input expressions can return a set.  Otherwise, invoke
+	 * ExecMakeFunctionResultNoSets.  In either case, change the evalfunc
+	 * pointer to go directly there on subsequent uses.
+	 */
+	if (fcache->func.fn_retset || expression_returns_set((Node *) func->args))
+	{
 		fcache->xprstate.evalfunc = (ExprStateEvalFunc) ExecMakeFunctionResult;
-
 		return ExecMakeFunctionResult(fcache, econtext, isNull, isDone);
+	}
+	else
+	{
+		fcache->xprstate.evalfunc = (ExprStateEvalFunc) ExecMakeFunctionResultNoSets;
+		return ExecMakeFunctionResultNoSets(fcache, econtext, isNull, isDone);
+	}
 }
 
 /* ----------------------------------------------------------------
@@ -2394,10 +2403,22 @@ ExecEvalOper(FuncExprState *fcache,
 	init_fcache(op->opfuncid, op->inputcollid, fcache,
 				econtext->ecxt_per_query_memory, true);
 
-	/* Go directly to ExecMakeFunctionResult on subsequent uses */
+	/*
+	 * We need to invoke ExecMakeFunctionResult if either the function itself
+	 * or any of its input expressions can return a set.  Otherwise, invoke
+	 * ExecMakeFunctionResultNoSets.  In either case, change the evalfunc
+	 * pointer to go directly there on subsequent uses.
+	 */
+	if (fcache->func.fn_retset || expression_returns_set((Node *) op->args))
+	{
 		fcache->xprstate.evalfunc = (ExprStateEvalFunc) ExecMakeFunctionResult;
-
 		return ExecMakeFunctionResult(fcache, econtext, isNull, isDone);
+	}
+	else
+	{
+		fcache->xprstate.evalfunc = (ExprStateEvalFunc) ExecMakeFunctionResultNoSets;
+		return ExecMakeFunctionResultNoSets(fcache, econtext, isNull, isDone);
+	}
 }
 
 /* ----------------------------------------------------------------

src/test/regress/expected/rangefuncs.out

@@ -1992,3 +1992,17 @@ select * from foobar();  -- fail
 ERROR:  function return row and query-specified return row do not match
 DETAIL:  Returned row contains 3 attributes, but query expects 2.
 drop function foobar();
+-- check behavior when a function's input sometimes returns a set (bug #8228)
+SELECT *,
+  lower(CASE WHEN id = 2 THEN (regexp_matches(str, '^0*([1-9]\d+)$'))[1]
+        ELSE str
+        END)
+FROM
+  (VALUES (1,''), (2,'0000000049404'), (3,'FROM 10000000876')) v(id, str);
+ id |       str        |      lower       
+----+------------------+------------------
+  1 |                  | 
+  2 | 0000000049404    | 49404
+  3 | FROM 10000000876 | from 10000000876
+(3 rows)
+

src/test/regress/sql/rangefuncs.sql

@@ -599,3 +599,12 @@ $$ select (1, 2.1, 3) $$ language sql;
 select * from foobar();  -- fail
 
 drop function foobar();
+
+-- check behavior when a function's input sometimes returns a set (bug #8228)
+
+SELECT *,
+  lower(CASE WHEN id = 2 THEN (regexp_matches(str, '^0*([1-9]\d+)$'))[1]
+        ELSE str
+        END)
+FROM
+  (VALUES (1,''), (2,'0000000049404'), (3,'FROM 10000000876')) v(id, str);