Generalize ri_RootToPartitionMap to use for non-partition children

ri_RootToPartitionMap is currently only initialized for tuple routing
target partitions, though a future commit will need the ability to use
it even for the non-partition child tables, so make adjustments to the
decouple it from the partitioning code.

Also, make it lazily initialized via ExecGetRootToChildMap(), making
that function its preferred access path.  Existing third-party code
accessing it directly should no longer do so; consequently, it's been
renamed to ri_RootToChildMap, which also makes it consistent with
ri_ChildToRootMap.

ExecGetRootToChildMap() houses the logic of setting the map appropriately
depending on whether a given child relation is partition or not.

To support this, also add a separate entry point for TupleConversionMap
creation that receives an AttrMap.  No new code here, just split an
existing function in two.

Author: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CA+HiwqEYUhDXSK5BTvG_xk=eaAEJCD4GS3C6uH7ybBvv+Z_Tmg@mail.gmail.com

src/backend/executor/execMain.c

@@ -1256,9 +1256,11 @@ InitResultRelInfo(ResultRelInfo *resultRelInfo,
 	 * this field is filled in ExecInitModifyTable().
 	 */
 	resultRelInfo->ri_RootResultRelInfo = partition_root_rri;
-	resultRelInfo->ri_RootToPartitionMap = NULL;	/* set by
-													 * ExecInitRoutingInfo */
-	resultRelInfo->ri_PartitionTupleSlot = NULL;	/* ditto */
+	/* Set by ExecGetRootToChildMap */
+	resultRelInfo->ri_RootToChildMap = NULL;
+	resultRelInfo->ri_RootToChildMapValid = false;
+	/* Set by ExecInitRoutingInfo */
+	resultRelInfo->ri_PartitionTupleSlot = NULL;
 	resultRelInfo->ri_ChildToRootMap = NULL;
 	resultRelInfo->ri_ChildToRootMapValid = false;
 	resultRelInfo->ri_CopyMultiInsertBuffer = NULL;

src/backend/executor/execPartition.c

@@ -463,7 +463,7 @@ ExecFindPartition(ModifyTableState *mtstate,
 			 */
 			if (is_leaf)
 			{
-				TupleConversionMap *map = rri->ri_RootToPartitionMap;
+				TupleConversionMap *map = ExecGetRootToChildMap(rri, estate);
 
 				if (map)
 					slot = execute_attr_map_slot(map->attrMap, rootslot,
@@ -727,7 +727,7 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 			OnConflictSetState *onconfl = makeNode(OnConflictSetState);
 			TupleConversionMap *map;
 
-			map = leaf_part_rri->ri_RootToPartitionMap;
+			map = ExecGetRootToChildMap(leaf_part_rri, estate);
 
 			Assert(node->onConflictSet != NIL);
 			Assert(rootResultRelInfo->ri_onConflict != NULL);
@@ -977,33 +977,24 @@ ExecInitRoutingInfo(ModifyTableState *mtstate,
 					int partidx,
 					bool is_borrowed_rel)
 {
-	ResultRelInfo *rootRelInfo = partRelInfo->ri_RootResultRelInfo;
 	MemoryContext oldcxt;
 	int			rri_index;
 
 	oldcxt = MemoryContextSwitchTo(proute->memcxt);
 
 	/*
-	 * Set up a tuple conversion map to convert a tuple routed to the
-	 * partition from the parent's type to the partition's.
+	 * Set up tuple conversion between root parent and the partition if the
+	 * two have different rowtypes.  If conversion is indeed required, also
+	 * initialize a slot dedicated to storing this partition's converted
+	 * tuples.  Various operations that are applied to tuples after routing,
+	 * such as checking constraints, will refer to this slot.
 	 */
-	partRelInfo->ri_RootToPartitionMap =
-		convert_tuples_by_name(RelationGetDescr(rootRelInfo->ri_RelationDesc),
-							   RelationGetDescr(partRelInfo->ri_RelationDesc));
-
-	/*
-	 * If a partition has a different rowtype than the root parent, initialize
-	 * a slot dedicated to storing this partition's tuples.  The slot is used
-	 * for various operations that are applied to tuples after routing, such
-	 * as checking constraints.
-	 */
-	if (partRelInfo->ri_RootToPartitionMap != NULL)
+	if (ExecGetRootToChildMap(partRelInfo, estate) != NULL)
 	{
 		Relation	partrel = partRelInfo->ri_RelationDesc;
 
 		/*
-		 * Initialize the slot itself setting its descriptor to this
-		 * partition's TupleDesc; TupleDesc reference will be released at the
+		 * This pins the partition's TupleDesc, which will be released at the
 		 * end of the command.
 		 */
 		partRelInfo->ri_PartitionTupleSlot =

src/backend/executor/execUtils.c

@@ -1253,6 +1253,45 @@ ExecGetChildToRootMap(ResultRelInfo *resultRelInfo)
 	return resultRelInfo->ri_ChildToRootMap;
 }
 
+/*
+ * Returns the map needed to convert given root result relation's tuples to
+ * the rowtype of the given child relation.  Note that a NULL result is valid
+ * and means that no conversion is needed.
+ */
+TupleConversionMap *
+ExecGetRootToChildMap(ResultRelInfo *resultRelInfo, EState *estate)
+{
+	/* Mustn't get called for a non-child result relation. */
+	Assert(resultRelInfo->ri_RootResultRelInfo);
+
+	/* If we didn't already do so, compute the map for this child. */
+	if (!resultRelInfo->ri_RootToChildMapValid)
+	{
+		ResultRelInfo *rootRelInfo = resultRelInfo->ri_RootResultRelInfo;
+		TupleDesc	indesc = RelationGetDescr(rootRelInfo->ri_RelationDesc);
+		TupleDesc	outdesc = RelationGetDescr(resultRelInfo->ri_RelationDesc);
+		Relation	childrel = resultRelInfo->ri_RelationDesc;
+		AttrMap    *attrMap;
+		MemoryContext oldcontext;
+
+		/*
+		 * When this child table is not a partition (!relispartition), it may
+		 * have columns that are not present in the root table, which we ask
+		 * to ignore by passing true for missing_ok.
+		 */
+		oldcontext = MemoryContextSwitchTo(estate->es_query_cxt);
+		attrMap = build_attrmap_by_name_if_req(indesc, outdesc,
+											   !childrel->rd_rel->relispartition);
+		if (attrMap)
+			resultRelInfo->ri_RootToChildMap =
+				convert_tuples_by_name_attrmap(indesc, outdesc, attrMap);
+		MemoryContextSwitchTo(oldcontext);
+		resultRelInfo->ri_RootToChildMapValid = true;
+	}
+
+	return resultRelInfo->ri_RootToChildMap;
+}
+
 /* Return a bitmap representing columns being inserted */
 Bitmapset *
 ExecGetInsertedCols(ResultRelInfo *relinfo, EState *estate)
@@ -1273,10 +1312,10 @@ ExecGetInsertedCols(ResultRelInfo *relinfo, EState *estate)
 	{
 		ResultRelInfo *rootRelInfo = relinfo->ri_RootResultRelInfo;
 		RangeTblEntry *rte = exec_rt_fetch(rootRelInfo->ri_RangeTableIndex, estate);
+		TupleConversionMap *map = ExecGetRootToChildMap(relinfo, estate);
 
-		if (relinfo->ri_RootToPartitionMap != NULL)
-			return execute_attr_map_cols(relinfo->ri_RootToPartitionMap->attrMap,
-										 rte->insertedCols);
+		if (map != NULL)
+			return execute_attr_map_cols(map->attrMap, rte->insertedCols);
 		else
 			return rte->insertedCols;
 	}
@@ -1307,10 +1346,10 @@ ExecGetUpdatedCols(ResultRelInfo *relinfo, EState *estate)
 	{
 		ResultRelInfo *rootRelInfo = relinfo->ri_RootResultRelInfo;
 		RangeTblEntry *rte = exec_rt_fetch(rootRelInfo->ri_RangeTableIndex, estate);
+		TupleConversionMap *map = ExecGetRootToChildMap(relinfo, estate);
 
-		if (relinfo->ri_RootToPartitionMap != NULL)
-			return execute_attr_map_cols(relinfo->ri_RootToPartitionMap->attrMap,
-										 rte->updatedCols);
+		if (map != NULL)
+			return execute_attr_map_cols(map->attrMap, rte->updatedCols);
 		else
 			return rte->updatedCols;
 	}
@@ -1333,10 +1372,10 @@ ExecGetExtraUpdatedCols(ResultRelInfo *relinfo, EState *estate)
 	{
 		ResultRelInfo *rootRelInfo = relinfo->ri_RootResultRelInfo;
 		RangeTblEntry *rte = exec_rt_fetch(rootRelInfo->ri_RangeTableIndex, estate);
+		TupleConversionMap *map = ExecGetRootToChildMap(relinfo, estate);
 
-		if (relinfo->ri_RootToPartitionMap != NULL)
-			return execute_attr_map_cols(relinfo->ri_RootToPartitionMap->attrMap,
-										 rte->extraUpdatedCols);
+		if (map != NULL)
+			return execute_attr_map_cols(map->attrMap, rte->extraUpdatedCols);
 		else
 			return rte->extraUpdatedCols;
 	}

src/backend/executor/nodeModifyTable.c

@@ -3481,7 +3481,7 @@ ExecPrepareTupleRouting(ModifyTableState *mtstate,
 	/*
 	 * Convert the tuple, if necessary.
 	 */
-	map = partrel->ri_RootToPartitionMap;
+	map = ExecGetRootToChildMap(partrel, estate);
 	if (map != NULL)
 	{
 		TupleTableSlot *new_slot = partrel->ri_PartitionTupleSlot;