Do execGrouping.c via expression eval machinery.

This has a performance benefit on own, although not hugely so. The
primary benefit is that it will allow for to JIT tuple deforming and
comparator invocations.

Author: Andres Freund
Discussion: https://postgr.es/m/20171129080934.amqqkke2zjtekd4t@alap3.anarazel.de

src/backend/executor/nodeSubplan.c

@@ -494,7 +494,9 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
 	if (nbuckets < 1)
 		nbuckets = 1;
 
-	node->hashtable = BuildTupleHashTable(ncols,
+	node->hashtable = BuildTupleHashTable(node->parent,
+										  node->descRight,
+										  ncols,
 										  node->keyColIdx,
 										  node->tab_eq_funcs,
 										  node->tab_hash_funcs,
@@ -514,7 +516,9 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
 			if (nbuckets < 1)
 				nbuckets = 1;
 		}
-		node->hashnulls = BuildTupleHashTable(ncols,
+		node->hashnulls = BuildTupleHashTable(node->parent,
+											  node->descRight,
+											  ncols,
 											  node->keyColIdx,
 											  node->tab_eq_funcs,
 											  node->tab_hash_funcs,
@@ -598,6 +602,78 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
 	MemoryContextSwitchTo(oldcontext);
 }
 
+
+/*
+ * execTuplesUnequal
+ *		Return true if two tuples are definitely unequal in the indicated
+ *		fields.
+ *
+ * Nulls are neither equal nor unequal to anything else.  A true result
+ * is obtained only if there are non-null fields that compare not-equal.
+ *
+ * slot1, slot2: the tuples to compare (must have same columns!)
+ * numCols: the number of attributes to be examined
+ * matchColIdx: array of attribute column numbers
+ * eqFunctions: array of fmgr lookup info for the equality functions to use
+ * evalContext: short-term memory context for executing the functions
+ */
+static bool
+execTuplesUnequal(TupleTableSlot *slot1,
+				  TupleTableSlot *slot2,
+				  int numCols,
+				  AttrNumber *matchColIdx,
+				  FmgrInfo *eqfunctions,
+				  MemoryContext evalContext)
+{
+	MemoryContext oldContext;
+	bool		result;
+	int			i;
+
+	/* Reset and switch into the temp context. */
+	MemoryContextReset(evalContext);
+	oldContext = MemoryContextSwitchTo(evalContext);
+
+	/*
+	 * We cannot report a match without checking all the fields, but we can
+	 * report a non-match as soon as we find unequal fields.  So, start
+	 * comparing at the last field (least significant sort key). That's the
+	 * most likely to be different if we are dealing with sorted input.
+	 */
+	result = false;
+
+	for (i = numCols; --i >= 0;)
+	{
+		AttrNumber	att = matchColIdx[i];
+		Datum		attr1,
+					attr2;
+		bool		isNull1,
+					isNull2;
+
+		attr1 = slot_getattr(slot1, att, &isNull1);
+
+		if (isNull1)
+			continue;			/* can't prove anything here */
+
+		attr2 = slot_getattr(slot2, att, &isNull2);
+
+		if (isNull2)
+			continue;			/* can't prove anything here */
+
+		/* Apply the type-specific equality function */
+
+		if (!DatumGetBool(FunctionCall2(&eqfunctions[i],
+										attr1, attr2)))
+		{
+			result = true;		/* they are unequal */
+			break;
+		}
+	}
+
+	MemoryContextSwitchTo(oldContext);
+
+	return result;
+}
+
 /*
  * findPartialMatch: does the hashtable contain an entry that is not
  * provably distinct from the tuple?
@@ -887,6 +963,7 @@ ExecInitSubPlan(SubPlan *subplan, PlanState *parent)
 												   NULL);
 
 		tupDesc = ExecTypeFromTL(righttlist, false);
+		sstate->descRight = tupDesc;
 		slot = ExecInitExtraTupleSlot(estate);
 		ExecSetSlotDescriptor(slot, tupDesc);
 		sstate->projRight = ExecBuildProjectionInfo(righttlist,