Fix mishandling of resjunk columns in ON CONFLICT ... UPDATE tlists.

It's unusual to have any resjunk columns in an ON CONFLICT ... UPDATE
list, but it can happen when MULTIEXPR_SUBLINK SubPlans are present.
If it happens, the ON CONFLICT UPDATE code path would end up storing
tuples that include the values of the extra resjunk columns.  That's
fairly harmless in the short run, but if new columns are added to
the table then the values would become accessible, possibly leading
to malfunctions if they don't match the datatypes of the new columns.

This had escaped notice through a confluence of missing sanity checks,
including

* There's no cross-check that a tuple presented to heap_insert or
heap_update matches the table rowtype.  While it's difficult to
check that fully at reasonable cost, we can easily add assertions
that there aren't too many columns.

* The output-column-assignment cases in execExprInterp.c lacked
any sanity checks on the output column numbers, which seems like
an oversight considering there are plenty of assertion checks on
input column numbers.  Add assertions there too.

* We failed to apply nodeModifyTable's ExecCheckPlanOutput() to
the ON CONFLICT UPDATE tlist.  That wouldn't have caught this
specific error, since that function is chartered to ignore resjunk
columns; but it sure seems like a bad omission now that we've seen
this bug.

In HEAD, the right way to fix this is to make the processing of
ON CONFLICT UPDATE tlists work the same as regular UPDATE tlists
now do, that is don't add "SET x = x" entries, and use
ExecBuildUpdateProjection to evaluate the tlist and combine it with
old values of the not-set columns.  This adds a little complication
to ExecBuildUpdateProjection, but allows removal of a comparable
amount of now-dead code from the planner.

In the back branches, the most expedient solution seems to be to
(a) use an output slot for the ON CONFLICT UPDATE projection that
actually matches the target table, and then (b) invent a variant of
ExecBuildProjectionInfo that can be told to not store values resulting
from resjunk columns, so it doesn't try to store into nonexistent
columns of the output slot.  (We can't simply ignore the resjunk columns
altogether; they have to be evaluated for MULTIEXPR_SUBLINK to work.)
This works back to v10.  In 9.6, projections work much differently and
we can't cheaply give them such an option.  The 9.6 version of this
patch works by inserting a JunkFilter when it's necessary to get rid
of resjunk columns.

In addition, v11 and up have the reverse problem when trying to
perform ON CONFLICT UPDATE on a partitioned table.  Through a
further oversight, adjust_partition_tlist() discarded resjunk columns
when re-ordering the ON CONFLICT UPDATE tlist to match a partition.
This accidentally prevented the storing-bogus-tuples problem, but
at the cost that MULTIEXPR_SUBLINK cases didn't work, typically
crashing if more than one row has to be updated.  Fix by preserving
resjunk columns in that routine.  (I failed to resist the temptation
to add more assertions there too, and to do some minor code
beautification.)

Per report from Andres Freund.  Back-patch to all supported branches.

Security: CVE-2021-32028

10 files changed, 196 insertions, 194 deletions

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 13396eb7f2..ba36da2b83 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -2070,6 +2070,10 @@ heap_insert(Relation relation, HeapTuple tup, CommandId cid,
 	bool		all_frozen_set = false;
 	uint8		vmstatus = 0;
 
+	/* Cheap, simplistic check that the tuple matches the rel's rowtype. */
+	Assert(HeapTupleHeaderGetNatts(tup->t_data) <=
+		   RelationGetNumberOfAttributes(relation));
+
 	/*
 	 * Fill in tuple header fields and toast the tuple if necessary.
 	 *
@@ -3255,6 +3259,10 @@ heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
 
 	Assert(ItemPointerIsValid(otid));
 
+	/* Cheap, simplistic check that the tuple matches the rel's rowtype. */
+	Assert(HeapTupleHeaderGetNatts(newtup->t_data) <=
+		   RelationGetNumberOfAttributes(relation));
+
 	/*
 	 * Forbid this during a parallel operation, lest it allocate a combo CID.
 	 * Other workers might need that combo CID for visibility checks, and we
diff --git a/src/backend/executor/execExpr.c b/src/backend/executor/execExpr.c
index 77c9d785d9..8c9f8a6aeb 100644
--- a/src/backend/executor/execExpr.c
+++ b/src/backend/executor/execExpr.c
@@ -485,14 +485,21 @@ ExecBuildProjectionInfo(List *targetList,
  * be stored into the given tuple slot.  (Caller must have ensured that tuple
  * slot has a descriptor matching the target rel!)
  *
- * subTargetList is the tlist of the subplan node feeding ModifyTable.
- * We use this mainly to cross-check that the expressions being assigned
- * are of the correct types.  The values from this tlist are assumed to be
- * available from the "outer" tuple slot.  They are assigned to target columns
- * listed in the corresponding targetColnos elements.  (Only non-resjunk tlist
- * entries are assigned.)  Columns not listed in targetColnos are filled from
- * the UPDATE's old tuple, which is assumed to be available in the "scan"
- * tuple slot.
+ * When evalTargetList is false, targetList contains the UPDATE ... SET
+ * expressions that have already been computed by a subplan node; the values
+ * from this tlist are assumed to be available in the "outer" tuple slot.
+ * When evalTargetList is true, targetList contains the UPDATE ... SET
+ * expressions that must be computed (which could contain references to
+ * the outer, inner, or scan tuple slots).
+ *
+ * In either case, targetColnos contains a list of the target column numbers
+ * corresponding to the non-resjunk entries of targetList.  The tlist values
+ * are assigned into these columns of the result tuple slot.  Target columns
+ * not listed in targetColnos are filled from the UPDATE's old tuple, which
+ * is assumed to be available in the "scan" tuple slot.
+ *
+ * targetList can also contain resjunk columns.  These must be evaluated
+ * if evalTargetList is true, but their values are discarded.
  *
  * relDesc must describe the relation we intend to update.
  *
@@ -503,7 +510,8 @@ ExecBuildProjectionInfo(List *targetList,
  * ExecCheckPlanOutput, so we must do our safety checks here.
  */
 ProjectionInfo *
-ExecBuildUpdateProjection(List *subTargetList,
+ExecBuildUpdateProjection(List *targetList,
+						  bool evalTargetList,
 						  List *targetColnos,
 						  TupleDesc relDesc,
 						  ExprContext *econtext,
@@ -525,19 +533,22 @@ ExecBuildUpdateProjection(List *subTargetList,
 	/* We embed ExprState into ProjectionInfo instead of doing extra palloc */
 	projInfo->pi_state.tag = T_ExprState;
 	state = &projInfo->pi_state;
-	state->expr = NULL;			/* not used */
+	if (evalTargetList)
+		state->expr = (Expr *) targetList;
+	else
+		state->expr = NULL;		/* not used */
 	state->parent = parent;
 	state->ext_params = NULL;
 
 	state->resultslot = slot;
 
 	/*
-	 * Examine the subplan tlist to see how many non-junk columns there are,
-	 * and to verify that the non-junk columns come before the junk ones.
+	 * Examine the targetList to see how many non-junk columns there are, and
+	 * to verify that the non-junk columns come before the junk ones.
 	 */
 	nAssignableCols = 0;
 	sawJunk = false;
-	foreach(lc, subTargetList)
+	foreach(lc, targetList)
 	{
 		TargetEntry *tle = lfirst_node(TargetEntry, lc);
 
@@ -569,12 +580,10 @@ ExecBuildUpdateProjection(List *subTargetList,
 	}
 
 	/*
-	 * We want to insert EEOP_*_FETCHSOME steps to ensure the outer and scan
-	 * tuples are sufficiently deconstructed.  Outer tuple is easy, but for
-	 * scan tuple we must find out the last old column we need.
+	 * We need to insert EEOP_*_FETCHSOME steps to ensure the input tuples are
+	 * sufficiently deconstructed.  The scan tuple must be deconstructed at
+	 * least as far as the last old column we need.
 	 */
-	deform.last_outer = nAssignableCols;
-
 	for (int attnum = relDesc->natts; attnum > 0; attnum--)
 	{
 		Form_pg_attribute attr = TupleDescAttr(relDesc, attnum - 1);
@@ -587,15 +596,26 @@ ExecBuildUpdateProjection(List *subTargetList,
 		break;
 	}
 
+	/*
+	 * If we're actually evaluating the tlist, incorporate its input
+	 * requirements too; otherwise, we'll just need to fetch the appropriate
+	 * number of columns of the "outer" tuple.
+	 */
+	if (evalTargetList)
+		get_last_attnums_walker((Node *) targetList, &deform);
+	else
+		deform.last_outer = nAssignableCols;
+
 	ExecPushExprSlots(state, &deform);
 
 	/*
-	 * Now generate code to fetch data from the outer tuple, incidentally
-	 * validating that it'll be of the right type.  The checks above ensure
-	 * that the forboth() will iterate over exactly the non-junk columns.
+	 * Now generate code to evaluate the tlist's assignable expressions or
+	 * fetch them from the outer tuple, incidentally validating that they'll
+	 * be of the right data type.  The checks above ensure that the forboth()
+	 * will iterate over exactly the non-junk columns.
 	 */
 	outerattnum = 0;
-	forboth(lc, subTargetList, lc2, targetColnos)
+	forboth(lc, targetList, lc2, targetColnos)
 	{
 		TargetEntry *tle = lfirst_node(TargetEntry, lc);
 		AttrNumber	targetattnum = lfirst_int(lc2);
@@ -628,13 +648,47 @@ ExecBuildUpdateProjection(List *subTargetList,
 							   targetattnum,
 							   format_type_be(exprType((Node *) tle->expr)))));
 
-		/*
-		 * OK, build an outer-tuple reference.
-		 */
-		scratch.opcode = EEOP_ASSIGN_OUTER_VAR;
-		scratch.d.assign_var.attnum = outerattnum++;
-		scratch.d.assign_var.resultnum = targetattnum - 1;
-		ExprEvalPushStep(state, &scratch);
+		/* OK, generate code to perform the assignment. */
+		if (evalTargetList)
+		{
+			/*
+			 * We must evaluate the TLE's expression and assign it.  We do not
+			 * bother jumping through hoops for "safe" Vars like
+			 * ExecBuildProjectionInfo does; this is a relatively less-used
+			 * path and it doesn't seem worth expending code for that.
+			 */
+			ExecInitExprRec(tle->expr, state,
+							&state->resvalue, &state->resnull);
+			/* Needn't worry about read-only-ness here, either. */
+			scratch.opcode = EEOP_ASSIGN_TMP;
+			scratch.d.assign_tmp.resultnum = targetattnum - 1;
+			ExprEvalPushStep(state, &scratch);
+		}
+		else
+		{
+			/* Just assign from the outer tuple. */
+			scratch.opcode = EEOP_ASSIGN_OUTER_VAR;
+			scratch.d.assign_var.attnum = outerattnum;
+			scratch.d.assign_var.resultnum = targetattnum - 1;
+			ExprEvalPushStep(state, &scratch);
+		}
+		outerattnum++;
+	}
+
+	/*
+	 * If we're evaluating the tlist, must evaluate any resjunk columns too.
+	 * (This matters for things like MULTIEXPR_SUBLINK SubPlans.)
+	 */
+	if (evalTargetList)
+	{
+		for_each_cell(lc, targetList, lc)
+		{
+			TargetEntry *tle = lfirst_node(TargetEntry, lc);
+
+			Assert(tle->resjunk);
+			ExecInitExprRec(tle->expr, state,
+							&state->resvalue, &state->resnull);
+		}
 	}
 
 	/*
diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c
index a9ed98ae48..5483dee650 100644
--- a/src/backend/executor/execExprInterp.c
+++ b/src/backend/executor/execExprInterp.c
@@ -626,6 +626,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			 * care of at compilation time.  But see EEOP_INNER_VAR comments.
 			 */
 			Assert(attnum >= 0 && attnum < innerslot->tts_nvalid);
+			Assert(resultnum >= 0 && resultnum < resultslot->tts_tupleDescriptor->natts);
 			resultslot->tts_values[resultnum] = innerslot->tts_values[attnum];
 			resultslot->tts_isnull[resultnum] = innerslot->tts_isnull[attnum];
 
@@ -642,6 +643,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			 * care of at compilation time.  But see EEOP_INNER_VAR comments.
 			 */
 			Assert(attnum >= 0 && attnum < outerslot->tts_nvalid);
+			Assert(resultnum >= 0 && resultnum < resultslot->tts_tupleDescriptor->natts);
 			resultslot->tts_values[resultnum] = outerslot->tts_values[attnum];
 			resultslot->tts_isnull[resultnum] = outerslot->tts_isnull[attnum];
 
@@ -658,6 +660,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			 * care of at compilation time.  But see EEOP_INNER_VAR comments.
 			 */
 			Assert(attnum >= 0 && attnum < scanslot->tts_nvalid);
+			Assert(resultnum >= 0 && resultnum < resultslot->tts_tupleDescriptor->natts);
 			resultslot->tts_values[resultnum] = scanslot->tts_values[attnum];
 			resultslot->tts_isnull[resultnum] = scanslot->tts_isnull[attnum];
 
@@ -668,6 +671,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 		{
 			int			resultnum = op->d.assign_tmp.resultnum;
 
+			Assert(resultnum >= 0 && resultnum < resultslot->tts_tupleDescriptor->natts);
 			resultslot->tts_values[resultnum] = state->resvalue;
 			resultslot->tts_isnull[resultnum] = state->resnull;
 
@@ -678,6 +682,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 		{
 			int			resultnum = op->d.assign_tmp.resultnum;
 
+			Assert(resultnum >= 0 && resultnum < resultslot->tts_tupleDescriptor->natts);
 			resultslot->tts_isnull[resultnum] = state->resnull;
 			if (!resultslot->tts_isnull[resultnum])
 				resultslot->tts_values[resultnum] =
@@ -2091,8 +2096,10 @@ ExecJustAssignVarImpl(ExprState *state, TupleTableSlot *inslot, bool *isnull)
 	 *
 	 * Since we use slot_getattr(), we don't need to implement the FETCHSOME
 	 * step explicitly, and we also needn't Assert that the attnum is in range
-	 * --- slot_getattr() will take care of any problems.
+	 * --- slot_getattr() will take care of any problems.  Nonetheless, check
+	 * that resultnum is in range.
 	 */
+	Assert(resultnum >= 0 && resultnum < outslot->tts_tupleDescriptor->natts);
 	outslot->tts_values[resultnum] =
 		slot_getattr(inslot, attnum, &outslot->tts_isnull[resultnum]);
 	return 0;
@@ -2224,6 +2231,7 @@ ExecJustAssignVarVirtImpl(ExprState *state, TupleTableSlot *inslot, bool *isnull
 	Assert(TTS_IS_VIRTUAL(inslot));
 	Assert(TTS_FIXED(inslot));
 	Assert(attnum >= 0 && attnum < inslot->tts_nvalid);
+	Assert(resultnum >= 0 && resultnum < outslot->tts_tupleDescriptor->natts);
 
 	outslot->tts_values[resultnum] = inslot->tts_values[attnum];
 	outslot->tts_isnull[resultnum] = inslot->tts_isnull[attnum];
diff --git a/src/backend/executor/execPartition.c b/src/backend/executor/execPartition.c
index 8afddca73a..8e2feafd28 100644
--- a/src/backend/executor/execPartition.c
+++ b/src/backend/executor/execPartition.c
@@ -181,7 +181,7 @@ static char *ExecBuildSlotPartitionKeyDescription(Relation rel,
 												  Datum *values,
 												  bool *isnull,
 												  int maxfieldlen);
-static List *adjust_partition_tlist(List *tlist, TupleConversionMap *map);
+static List *adjust_partition_colnos(List *colnos, ResultRelInfo *leaf_part_rri);
 static void ExecInitPruningContext(PartitionPruneContext *context,
 								   List *pruning_steps,
 								   PartitionDesc partdesc,
@@ -714,6 +714,7 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 		 */
 		if (node->onConflictAction == ONCONFLICT_UPDATE)
 		{
+			OnConflictSetState *onconfl = makeNode(OnConflictSetState);
 			TupleConversionMap *map;
 
 			map = leaf_part_rri->ri_RootToPartitionMap;
@@ -721,14 +722,14 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 			Assert(node->onConflictSet != NIL);
 			Assert(rootResultRelInfo->ri_onConflict != NULL);
 
-			leaf_part_rri->ri_onConflict = makeNode(OnConflictSetState);
+			leaf_part_rri->ri_onConflict = onconfl;
 
 			/*
 			 * Need a separate existing slot for each partition, as the
 			 * partition could be of a different AM, even if the tuple
 			 * descriptors match.
 			 */
-			leaf_part_rri->ri_onConflict->oc_Existing =
+			onconfl->oc_Existing =
 				table_slot_create(leaf_part_rri->ri_RelationDesc,
 								  &mtstate->ps.state->es_tupleTable);
 
@@ -748,17 +749,17 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 				 * Projections and where clauses themselves don't store state
 				 * / are independent of the underlying storage.
 				 */
-				leaf_part_rri->ri_onConflict->oc_ProjSlot =
+				onconfl->oc_ProjSlot =
 					rootResultRelInfo->ri_onConflict->oc_ProjSlot;
-				leaf_part_rri->ri_onConflict->oc_ProjInfo =
+				onconfl->oc_ProjInfo =
 					rootResultRelInfo->ri_onConflict->oc_ProjInfo;
-				leaf_part_rri->ri_onConflict->oc_WhereClause =
+				onconfl->oc_WhereClause =
 					rootResultRelInfo->ri_onConflict->oc_WhereClause;
 			}
 			else
 			{
 				List	   *onconflset;
-				TupleDesc	tupDesc;
+				List	   *onconflcols;
 				bool		found_whole_row;
 
 				/*
@@ -768,7 +769,7 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 				 * pseudo-relation (INNER_VAR), and second to handle the main
 				 * target relation (firstVarno).
 				 */
-				onconflset = (List *) copyObject((Node *) node->onConflictSet);
+				onconflset = copyObject(node->onConflictSet);
 				if (part_attmap == NULL)
 					part_attmap =
 						build_attrmap_by_name(RelationGetDescr(partrel),
@@ -788,20 +789,24 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 										&found_whole_row);
 				/* We ignore the value of found_whole_row. */
 
-				/* Finally, adjust this tlist to match the partition. */
-				onconflset = adjust_partition_tlist(onconflset, map);
+				/* Finally, adjust the target colnos to match the partition. */
+				onconflcols = adjust_partition_colnos(node->onConflictCols,
+													  leaf_part_rri);
 
 				/* create the tuple slot for the UPDATE SET projection */
-				tupDesc = ExecTypeFromTL(onconflset);
-				leaf_part_rri->ri_onConflict->oc_ProjSlot =
-					ExecInitExtraTupleSlot(mtstate->ps.state, tupDesc,
-										   &TTSOpsVirtual);
+				onconfl->oc_ProjSlot =
+					table_slot_create(partrel,
+									  &mtstate->ps.state->es_tupleTable);
 
 				/* build UPDATE SET projection state */
-				leaf_part_rri->ri_onConflict->oc_ProjInfo =
-					ExecBuildProjectionInfo(onconflset, econtext,
-											leaf_part_rri->ri_onConflict->oc_ProjSlot,
-											&mtstate->ps, partrelDesc);
+				onconfl->oc_ProjInfo =
+					ExecBuildUpdateProjection(onconflset,
+											  true,
+											  onconflcols,
+											  partrelDesc,
+											  econtext,
+											  onconfl->oc_ProjSlot,
+											  &mtstate->ps);
 
 				/*
 				 * If there is a WHERE clause, initialize state where it will
@@ -828,7 +833,7 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 											RelationGetForm(partrel)->reltype,
 											&found_whole_row);
 					/* We ignore the value of found_whole_row. */
-					leaf_part_rri->ri_onConflict->oc_WhereClause =
+					onconfl->oc_WhereClause =
 						ExecInitQual((List *) clause, &mtstate->ps);
 				}
 			}
@@ -1421,71 +1426,35 @@ ExecBuildSlotPartitionKeyDescription(Relation rel,
 }
 
 /*
- * adjust_partition_tlist
- *		Adjust the targetlist entries for a given partition to account for
- *		attribute differences between parent and the partition
- *
- * The expressions have already been fixed, but here we fix the list to make
- * target resnos match the partition's attribute numbers.  This results in a
- * copy of the original target list in which the entries appear in resno
- * order, including both the existing entries (that may have their resno
- * changed in-place) and the newly added entries for columns that don't exist
- * in the parent.
- *
- * Scribbles on the input tlist, so callers must make sure to make a copy
- * before passing it to us.
+ * adjust_partition_colnos
+ *		Adjust the list of UPDATE target column numbers to account for
+ *		attribute differences between the parent and the partition.
  */
 static List *
-adjust_partition_tlist(List *tlist, TupleConversionMap *map)
+adjust_partition_colnos(List *colnos, ResultRelInfo *leaf_part_rri)
 {
-	List	   *new_tlist = NIL;
-	TupleDesc	tupdesc = map->outdesc;
-	AttrMap    *attrMap = map->attrMap;
-	AttrNumber	attrno;
-
-	Assert(tupdesc->natts == attrMap->maplen);
-	for (attrno = 1; attrno <= tupdesc->natts; attrno++)
-	{
-		Form_pg_attribute att_tup = TupleDescAttr(tupdesc, attrno - 1);
-		TargetEntry *tle;
-
-		if (attrMap->attnums[attrno - 1] != InvalidAttrNumber)
-		{
-			Assert(!att_tup->attisdropped);
-
-			/*
-			 * Use the corresponding entry from the parent's tlist, adjusting
-			 * the resno the match the partition's attno.
-			 */
-			tle = (TargetEntry *) list_nth(tlist, attrMap->attnums[attrno - 1] - 1);
-			tle->resno = attrno;
-		}
-		else
-		{
-			Const	   *expr;
+	List	   *new_colnos = NIL;
+	TupleConversionMap *map = ExecGetChildToRootMap(leaf_part_rri);
+	AttrMap    *attrMap;
+	ListCell   *lc;
 
-			/*
-			 * For a dropped attribute in the partition, generate a dummy
-			 * entry with resno matching the partition's attno.
-			 */
-			Assert(att_tup->attisdropped);
-			expr = makeConst(INT4OID,
-							 -1,
-							 InvalidOid,
-							 sizeof(int32),
-							 (Datum) 0,
-							 true,	/* isnull */
-							 true /* byval */ );
-			tle = makeTargetEntry((Expr *) expr,
-								  attrno,
-								  pstrdup(NameStr(att_tup->attname)),
-								  false);
-		}
+	Assert(map != NULL);		/* else we shouldn't be here */
+	attrMap = map->attrMap;
 
-		new_tlist = lappend(new_tlist, tle);
+	foreach(lc, colnos)
+	{
+		AttrNumber	parentattrno = lfirst_int(lc);
+
+		if (parentattrno <= 0 ||
+			parentattrno > attrMap->maplen ||
+			attrMap->attnums[parentattrno - 1] == 0)
+			elog(ERROR, "unexpected attno %d in target column list",
+				 parentattrno);
+		new_colnos = lappend_int(new_colnos,
+								 attrMap->attnums[parentattrno - 1]);
 	}
 
-	return new_tlist;
+	return new_colnos;
 }
 
 /*-------------------------------------------------------------------------
diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index c5a2a9a054..a62928ae7c 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -492,6 +492,7 @@ ExecInitUpdateProjection(ModifyTableState *mtstate,
 
 	resultRelInfo->ri_projectNew =
 		ExecBuildUpdateProjection(subplan->targetlist,
+								  false,	/* subplan did the evaluation */
 								  updateColnos,
 								  relDesc,
 								  mtstate->ps.ps_ExprContext,
@@ -2972,9 +2973,9 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 	 */
 	if (node->onConflictAction == ONCONFLICT_UPDATE)
 	{
+		OnConflictSetState *onconfl = makeNode(OnConflictSetState);
 		ExprContext *econtext;
 		TupleDesc	relationDesc;
-		TupleDesc	tupDesc;
 
 		/* already exists if created by RETURNING processing above */
 		if (mtstate->ps.ps_ExprContext == NULL)
@@ -2984,10 +2985,10 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 		relationDesc = resultRelInfo->ri_RelationDesc->rd_att;
 
 		/* create state for DO UPDATE SET operation */
-		resultRelInfo->ri_onConflict = makeNode(OnConflictSetState);
+		resultRelInfo->ri_onConflict = onconfl;
 
 		/* initialize slot for the existing tuple */
-		resultRelInfo->ri_onConflict->oc_Existing =
+		onconfl->oc_Existing =
 			table_slot_create(resultRelInfo->ri_RelationDesc,
 							  &mtstate->ps.state->es_tupleTable);
 
@@ -2997,17 +2998,19 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 		 * into the table, and for RETURNING processing - which may access
 		 * system attributes.
 		 */
-		tupDesc = ExecTypeFromTL((List *) node->onConflictSet);
-		resultRelInfo->ri_onConflict->oc_ProjSlot =
-			ExecInitExtraTupleSlot(mtstate->ps.state, tupDesc,
-								   table_slot_callbacks(resultRelInfo->ri_RelationDesc));
+		onconfl->oc_ProjSlot =
+			table_slot_create(resultRelInfo->ri_RelationDesc,
+							  &mtstate->ps.state->es_tupleTable);
 
 		/* build UPDATE SET projection state */
-		resultRelInfo->ri_onConflict->oc_ProjInfo =
-			ExecBuildProjectionInfo(node->onConflictSet, econtext,
-									resultRelInfo->ri_onConflict->oc_ProjSlot,
-									&mtstate->ps,
-									relationDesc);
+		onconfl->oc_ProjInfo =
+			ExecBuildUpdateProjection(node->onConflictSet,
+									  true,
+									  node->onConflictCols,
+									  relationDesc,
+									  econtext,
+									  onconfl->oc_ProjSlot,
+									  &mtstate->ps);
 
 		/* initialize state to evaluate the WHERE clause, if any */
 		if (node->onConflictWhere)
@@ -3016,7 +3019,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 
 			qualexpr = ExecInitQual((List *) node->onConflictWhere,
 									&mtstate->ps);
-			resultRelInfo->ri_onConflict->oc_WhereClause = qualexpr;
+			onconfl->oc_WhereClause = qualexpr;
 		}
 	}
 
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index f5a7760740..90770a89b0 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -217,6 +217,7 @@ _copyModifyTable(const ModifyTable *from)
 	COPY_SCALAR_FIELD(onConflictAction);
 	COPY_NODE_FIELD(arbiterIndexes);
 	COPY_NODE_FIELD(onConflictSet);
+	COPY_NODE_FIELD(onConflictCols);
 	COPY_NODE_FIELD(onConflictWhere);
 	COPY_SCALAR_FIELD(exclRelRTI);
 	COPY_NODE_FIELD(exclRelTlist);
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index c723f6d635..8da8b14f0e 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -418,6 +418,7 @@ _outModifyTable(StringInfo str, const ModifyTable *node)
 	WRITE_ENUM_FIELD(onConflictAction, OnConflictAction);
 	WRITE_NODE_FIELD(arbiterIndexes);
 	WRITE_NODE_FIELD(onConflictSet);
+	WRITE_NODE_FIELD(onConflictCols);
 	WRITE_NODE_FIELD(onConflictWhere);
 	WRITE_UINT_FIELD(exclRelRTI);
 	WRITE_NODE_FIELD(exclRelTlist);
diff --git a/src/backend/nodes/readfuncs.c b/src/backend/nodes/readfuncs.c
index 3746668f52..3772ea07df 100644
--- a/src/backend/nodes/readfuncs.c
+++ b/src/backend/nodes/readfuncs.c
@@ -1697,6 +1697,7 @@ _readModifyTable(void)
 	READ_ENUM_FIELD(onConflictAction, OnConflictAction);
 	READ_NODE_FIELD(arbiterIndexes);
 	READ_NODE_FIELD(onConflictSet);
+	READ_NODE_FIELD(onConflictCols);
 	READ_NODE_FIELD(onConflictWhere);
 	READ_UINT_FIELD(exclRelRTI);
 	READ_NODE_FIELD(exclRelTlist);
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index a9aff24831..7003238d76 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -34,6 +34,7 @@
 #include "optimizer/placeholder.h"
 #include "optimizer/plancat.h"
 #include "optimizer/planmain.h"
+#include "optimizer/prep.h"
 #include "optimizer/restrictinfo.h"
 #include "optimizer/subselect.h"
 #include "optimizer/tlist.h"
@@ -6909,6 +6910,7 @@ make_modifytable(PlannerInfo *root, Plan *subplan,
 	{
 		node->onConflictAction = ONCONFLICT_NONE;
 		node->onConflictSet = NIL;
+		node->onConflictCols = NIL;
 		node->onConflictWhere = NULL;
 		node->arbiterIndexes = NIL;
 		node->exclRelRTI = 0;
@@ -6917,7 +6919,16 @@ make_modifytable(PlannerInfo *root, Plan *subplan,
 	else
 	{
 		node->onConflictAction = onconflict->action;
+
+		/*
+		 * Here we convert the ON CONFLICT UPDATE tlist, if any, to the
+		 * executor's convention of having consecutive resno's.  The actual
+		 * target column numbers are saved in node->onConflictCols.  (This
+		 * could be done earlier, but there seems no need to.)
+		 */
 		node->onConflictSet = onconflict->onConflictSet;
+		node->onConflictCols =
+			extract_update_targetlist_colnos(node->onConflictSet);
 		node->onConflictWhere = onconflict->onConflictWhere;
 
 		/*
diff --git a/src/backend/optimizer/prep/preptlist.c b/src/backend/optimizer/prep/preptlist.c
index 363132185d..aefb6f8d4e 100644
--- a/src/backend/optimizer/prep/preptlist.c
+++ b/src/backend/optimizer/prep/preptlist.c
@@ -46,9 +46,7 @@
 #include "parser/parsetree.h"
 #include "utils/rel.h"
 
-static List *extract_update_colnos(List *tlist);
-static List *expand_targetlist(List *tlist, int command_type,
-							   Index result_relation, Relation rel);
+static List *expand_insert_targetlist(List *tlist, Relation rel);
 
 
 /*
@@ -59,9 +57,6 @@ static List *expand_targetlist(List *tlist, int command_type,
  * Also, if this is an UPDATE, we return a list of target column numbers
  * in root->update_colnos.  (Resnos in processed_tlist will be consecutive,
  * so do not look at that to find out which columns are targets!)
- *
- * As a side effect, if there's an ON CONFLICT UPDATE clause, its targetlist
- * is also preprocessed (and updated in-place).
  */
 void
 preprocess_targetlist(PlannerInfo *root)
@@ -107,10 +102,9 @@ preprocess_targetlist(PlannerInfo *root)
 	 */
 	tlist = parse->targetList;
 	if (command_type == CMD_INSERT)
-		tlist = expand_targetlist(tlist, command_type,
-								  result_relation, target_relation);
+		tlist = expand_insert_targetlist(tlist, target_relation);
 	else if (command_type == CMD_UPDATE)
-		root->update_colnos = extract_update_colnos(tlist);
+		root->update_colnos = extract_update_targetlist_colnos(tlist);
 
 	/*
 	 * For non-inherited UPDATE/DELETE, register any junk column(s) needed to
@@ -245,23 +239,12 @@ preprocess_targetlist(PlannerInfo *root)
 
 	root->processed_tlist = tlist;
 
-	/*
-	 * If there's an ON CONFLICT UPDATE clause, preprocess its targetlist too
-	 * while we have the relation open.
-	 */
-	if (parse->onConflict)
-		parse->onConflict->onConflictSet =
-			expand_targetlist(parse->onConflict->onConflictSet,
-							  CMD_UPDATE,
-							  result_relation,
-							  target_relation);
-
 	if (target_relation)
 		table_close(target_relation, NoLock);
 }
 
 /*
- * extract_update_colnos
+ * extract_update_targetlist_colnos
  * 		Extract a list of the target-table column numbers that
  * 		an UPDATE's targetlist wants to assign to, then renumber.
  *
@@ -270,9 +253,12 @@ preprocess_targetlist(PlannerInfo *root)
  * to assign to.  Here, we extract that info into a separate list, and
  * then convert the tlist to the sequential-numbering convention that's
  * used by all other query types.
+ *
+ * This is also applied to the tlist associated with INSERT ... ON CONFLICT
+ * ... UPDATE, although not till much later in planning.
  */
-static List *
-extract_update_colnos(List *tlist)
+List *
+extract_update_targetlist_colnos(List *tlist)
 {
 	List	   *update_colnos = NIL;
 	AttrNumber	nextresno = 1;
@@ -297,18 +283,16 @@ extract_update_colnos(List *tlist)
  *****************************************************************************/
 
 /*
- * expand_targetlist
+ * expand_insert_targetlist
  *	  Given a target list as generated by the parser and a result relation,
  *	  add targetlist entries for any missing attributes, and ensure the
  *	  non-junk attributes appear in proper field order.
  *
- * command_type is a bit of an archaism now: it's CMD_INSERT when we're
- * processing an INSERT, all right, but the only other use of this function
- * is for ON CONFLICT UPDATE tlists, for which command_type is CMD_UPDATE.
+ * Once upon a time we also did more or less this with UPDATE targetlists,
+ * but now this code is only applied to INSERT targetlists.
  */
 static List *
-expand_targetlist(List *tlist, int command_type,
-				  Index result_relation, Relation rel)
+expand_insert_targetlist(List *tlist, Relation rel)
 {
 	List	   *new_tlist = NIL;
 	ListCell   *tlist_item;
@@ -347,15 +331,11 @@ expand_targetlist(List *tlist, int command_type,
 			/*
 			 * Didn't find a matching tlist entry, so make one.
 			 *
-			 * For INSERT, generate a NULL constant.  (We assume the rewriter
-			 * would have inserted any available default value.) Also, if the
-			 * column isn't dropped, apply any domain constraints that might
-			 * exist --- this is to catch domain NOT NULL.
-			 *
-			 * For UPDATE, generate a Var reference to the existing value of
-			 * the attribute, so that it gets copied to the new tuple. But
-			 * generate a NULL for dropped columns (we want to drop any old
-			 * values).
+			 * INSERTs should insert NULL in this case.  (We assume the
+			 * rewriter would have inserted any available non-NULL default
+			 * value.)  Also, if the column isn't dropped, apply any domain
+			 * constraints that might exist --- this is to catch domain NOT
+			 * NULL.
 			 *
 			 * When generating a NULL constant for a dropped column, we label
 			 * it INT4 (any other guaranteed-to-exist datatype would do as
@@ -367,13 +347,9 @@ expand_targetlist(List *tlist, int command_type,
 			 * relation, however.
 			 */
 			Oid			atttype = att_tup->atttypid;
-			int32		atttypmod = att_tup->atttypmod;
 			Oid			attcollation = att_tup->attcollation;
 			Node	   *new_expr;
 
-			switch (command_type)
-			{
-				case CMD_INSERT:
 					if (!att_tup->attisdropped)
 					{
 						new_expr = (Node *) makeConst(atttype,
@@ -402,35 +378,6 @@ expand_targetlist(List *tlist, int command_type,
 													  true, /* isnull */
 													  true /* byval */ );
 					}
-					break;
-				case CMD_UPDATE:
-					if (!att_tup->attisdropped)
-					{
-						new_expr = (Node *) makeVar(result_relation,
-													attrno,
-													atttype,
-													atttypmod,
-													attcollation,
-													0);
-					}
-					else
-					{
-						/* Insert NULL for dropped column */
-						new_expr = (Node *) makeConst(INT4OID,
-													  -1,
-													  InvalidOid,
-													  sizeof(int32),
-													  (Datum) 0,
-													  true, /* isnull */
-													  true /* byval */ );
-					}
-					break;
-				default:
-					elog(ERROR, "unrecognized command_type: %d",
-						 (int) command_type);
-					new_expr = NULL;	/* keep compiler quiet */
-					break;
-			}
 
 			new_tle = makeTargetEntry((Expr *) new_expr,
 									  attrno,
@@ -445,9 +392,8 @@ expand_targetlist(List *tlist, int command_type,
 	 * The remaining tlist entries should be resjunk; append them all to the
 	 * end of the new tlist, making sure they have resnos higher than the last
 	 * real attribute.  (Note: although the rewriter already did such
-	 * renumbering, we have to do it again here in case we are doing an UPDATE
-	 * in a table with dropped columns, or an inheritance child table with
-	 * extra columns.)
+	 * renumbering, we have to do it again here in case we added NULL entries
+	 * above.)
 	 */
 	while (tlist_item)
 	{