diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c
index 790380051b..f901baf1ed 100644
--- a/src/backend/executor/execExprInterp.c
+++ b/src/backend/executor/execExprInterp.c
@@ -1742,6 +1742,15 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			 * anything.  Also, if transfn returned a pointer to a R/W
 			 * expanded object that is already a child of the aggcontext,
 			 * assume we can adopt that value without copying it.
+			 *
+			 * It's safe to compare newVal with pergroup->transValue without
+			 * regard for either being NULL, because ExecAggTransReparent()
+			 * takes care to set transValue to 0 when NULL. Otherwise we could
+			 * end up accidentally not reparenting, when the transValue has
+			 * the same numerical value as newValue, despite being NULL.  This
+			 * is a somewhat hot path, making it undesirable to instead solve
+			 * this with another branch for the common case of the transition
+			 * function returning its (modified) input argument.
 			 */
 			if (DatumGetPointer(newVal) != DatumGetPointer(pergroup->transValue))
 				newVal = ExecAggTransReparent(aggstate, pertrans,
@@ -4186,6 +4195,8 @@ ExecAggTransReparent(AggState *aggstate, AggStatePerTrans pertrans,
 					 Datum newValue, bool newValueIsNull,
 					 Datum oldValue, bool oldValueIsNull)
 {
+	Assert(newValue != oldValue);
+
 	if (!newValueIsNull)
 	{
 		MemoryContextSwitchTo(aggstate->curaggcontext->ecxt_per_tuple_memory);
@@ -4199,6 +4210,16 @@ ExecAggTransReparent(AggState *aggstate, AggStatePerTrans pertrans,
 								 pertrans->transtypeByVal,
 								 pertrans->transtypeLen);
 	}
+	else
+	{
+		/*
+		 * Ensure that AggStatePerGroup->transValue ends up being 0, so
+		 * callers can safely compare newValue/oldValue without having to
+		 * check their respective nullness.
+		 */
+		newValue = (Datum) 0;
+	}
+
 	if (!oldValueIsNull)
 	{
 		if (DatumIsReadWriteExpandedObject(oldValue,
diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index 98bee4c5a6..7b8cb91f04 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -221,6 +221,7 @@
 #include "catalog/pg_aggregate.h"
 #include "catalog/pg_proc.h"
 #include "catalog/pg_type.h"
+#include "executor/execExpr.h"
 #include "executor/executor.h"
 #include "executor/nodeAgg.h"
 #include "miscadmin.h"
@@ -626,33 +627,22 @@ advance_transition_function(AggState *aggstate,
 	 * first input, we don't need to do anything.  Also, if transfn returned a
 	 * pointer to a R/W expanded object that is already a child of the
 	 * aggcontext, assume we can adopt that value without copying it.
+	 *
+	 * It's safe to compare newVal with pergroup->transValue without
+	 * regard for either being NULL, because ExecAggTransReparent()
+	 * takes care to set transValue to 0 when NULL. Otherwise we could
+	 * end up accidentally not reparenting, when the transValue has
+	 * the same numerical value as newValue, despite being NULL.  This
+	 * is a somewhat hot path, making it undesirable to instead solve
+	 * this with another branch for the common case of the transition
+	 * function returning its (modified) input argument.
 	 */
 	if (!pertrans->transtypeByVal &&
 		DatumGetPointer(newVal) != DatumGetPointer(pergroupstate->transValue))
-	{
-		if (!fcinfo->isnull)
-		{
-			MemoryContextSwitchTo(aggstate->curaggcontext->ecxt_per_tuple_memory);
-			if (DatumIsReadWriteExpandedObject(newVal,
-											   false,
-											   pertrans->transtypeLen) &&
-				MemoryContextGetParent(DatumGetEOHP(newVal)->eoh_context) == CurrentMemoryContext)
-				 /* do nothing */ ;
-			else
-				newVal = datumCopy(newVal,
-								   pertrans->transtypeByVal,
-								   pertrans->transtypeLen);
-		}
-		if (!pergroupstate->transValueIsNull)
-		{
-			if (DatumIsReadWriteExpandedObject(pergroupstate->transValue,
-											   false,
-											   pertrans->transtypeLen))
-				DeleteExpandedObject(pergroupstate->transValue);
-			else
-				pfree(DatumGetPointer(pergroupstate->transValue));
-		}
-	}
+		newVal = ExecAggTransReparent(aggstate, pertrans,
+									  newVal, fcinfo->isnull,
+									  pergroupstate->transValue,
+									  pergroupstate->transValueIsNull);
 
 	pergroupstate->transValue = newVal;
 	pergroupstate->transValueIsNull = fcinfo->isnull;