Remove quick path in ExecInitPartitionInfo for equal tupdescs

I added this "optimization" on top of Amit Langote's 158b7bc6d7, but
the quick path is never taken because the partition uses a different
pg_type oid than its parent table (causing equalTupleDescs to return
false).  Changing that requires more analysis and is too considered
dangerous at this point in the cycle, so revert it.

We might make it work someday, but not for pg11.

Discussion: https://postgr.es/m/825031be-942c-8c24-6163-13c27f217a3d@lab.ntt.co.jp

src/backend/executor/execPartition.c

@@ -313,7 +313,6 @@ ExecInitPartitionInfo(ModifyTableState *mtstate,
 	MemoryContext oldContext;
 	AttrNumber *part_attnos = NULL;
 	bool		found_whole_row;
-	bool		equalTupdescs;
 
 	/*
 	 * We locked all the partitions in ExecSetupPartitionTupleRouting
@@ -361,10 +360,6 @@ ExecInitPartitionInfo(ModifyTableState *mtstate,
 						(node != NULL &&
 						 node->onConflictAction != ONCONFLICT_NONE));
 
-	/* if tuple descs are identical, we don't need to map the attrs */
-	equalTupdescs = equalTupleDescs(RelationGetDescr(partrel),
-									RelationGetDescr(firstResultRel));
-
 	/*
 	 * Build WITH CHECK OPTION constraints for the partition.  Note that we
 	 * didn't build the withCheckOptionList for partitions within the planner,
@@ -405,21 +400,18 @@ ExecInitPartitionInfo(ModifyTableState *mtstate,
 		/*
 		 * Convert Vars in it to contain this partition's attribute numbers.
 		 */
-		if (!equalTupdescs)
-		{
-			part_attnos =
-				convert_tuples_by_name_map(RelationGetDescr(partrel),
-										   RelationGetDescr(firstResultRel),
-										   gettext_noop("could not convert row type"));
-			wcoList = (List *)
-				map_variable_attnos((Node *) wcoList,
-									firstVarno, 0,
-									part_attnos,
-									RelationGetDescr(firstResultRel)->natts,
-									RelationGetForm(partrel)->reltype,
-									&found_whole_row);
-			/* We ignore the value of found_whole_row. */
-		}
+		part_attnos =
+			convert_tuples_by_name_map(RelationGetDescr(partrel),
+									   RelationGetDescr(firstResultRel),
+									   gettext_noop("could not convert row type"));
+		wcoList = (List *)
+			map_variable_attnos((Node *) wcoList,
+								firstVarno, 0,
+								part_attnos,
+								RelationGetDescr(firstResultRel)->natts,
+								RelationGetForm(partrel)->reltype,
+								&found_whole_row);
+		/* We ignore the value of found_whole_row. */
 
 		foreach(ll, wcoList)
 		{
@@ -464,25 +456,22 @@ ExecInitPartitionInfo(ModifyTableState *mtstate,
 		 */
 		returningList = linitial(node->returningLists);
 
-		if (!equalTupdescs)
-		{
-			/*
-			 * Convert Vars in it to contain this partition's attribute numbers.
-			 */
-			if (part_attnos == NULL)
-				part_attnos =
-					convert_tuples_by_name_map(RelationGetDescr(partrel),
-											   RelationGetDescr(firstResultRel),
-											   gettext_noop("could not convert row type"));
-			returningList = (List *)
-				map_variable_attnos((Node *) returningList,
-									firstVarno, 0,
-									part_attnos,
-									RelationGetDescr(firstResultRel)->natts,
-									RelationGetForm(partrel)->reltype,
-									&found_whole_row);
-			/* We ignore the value of found_whole_row. */
-		}
+		/*
+		 * Convert Vars in it to contain this partition's attribute numbers.
+		 */
+		if (part_attnos == NULL)
+			part_attnos =
+				convert_tuples_by_name_map(RelationGetDescr(partrel),
+										   RelationGetDescr(firstResultRel),
+										   gettext_noop("could not convert row type"));
+		returningList = (List *)
+			map_variable_attnos((Node *) returningList,
+								firstVarno, 0,
+								part_attnos,
+								RelationGetDescr(firstResultRel)->natts,
+								RelationGetForm(partrel)->reltype,
+								&found_whole_row);
+		/* We ignore the value of found_whole_row. */
 
 		leaf_part_rri->ri_returningList = returningList;
 
@@ -583,33 +572,30 @@ ExecInitPartitionInfo(ModifyTableState *mtstate,
 				 * target relation (firstVarno).
 				 */
 				onconflset = (List *) copyObject((Node *) node->onConflictSet);
-				if (!equalTupdescs)
-				{
-					if (part_attnos == NULL)
-						part_attnos =
-							convert_tuples_by_name_map(RelationGetDescr(partrel),
-													   RelationGetDescr(firstResultRel),
-													   gettext_noop("could not convert row type"));
-					onconflset = (List *)
-						map_variable_attnos((Node *) onconflset,
-											INNER_VAR, 0,
-											part_attnos,
-											RelationGetDescr(firstResultRel)->natts,
-											RelationGetForm(partrel)->reltype,
-											&found_whole_row);
-					/* We ignore the value of found_whole_row. */
-					onconflset = (List *)
-						map_variable_attnos((Node *) onconflset,
-											firstVarno, 0,
-											part_attnos,
-											RelationGetDescr(firstResultRel)->natts,
-											RelationGetForm(partrel)->reltype,
-											&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);
-				}
+				if (part_attnos == NULL)
+					part_attnos =
+						convert_tuples_by_name_map(RelationGetDescr(partrel),
+												   RelationGetDescr(firstResultRel),
+												   gettext_noop("could not convert row type"));
+				onconflset = (List *)
+					map_variable_attnos((Node *) onconflset,
+										INNER_VAR, 0,
+										part_attnos,
+										RelationGetDescr(firstResultRel)->natts,
+										RelationGetForm(partrel)->reltype,
+										&found_whole_row);
+				/* We ignore the value of found_whole_row. */
+				onconflset = (List *)
+					map_variable_attnos((Node *) onconflset,
+										firstVarno, 0,
+										part_attnos,
+										RelationGetDescr(firstResultRel)->natts,
+										RelationGetForm(partrel)->reltype,
+										&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);
 
 				/*
 				 * Build UPDATE SET's projection info.  The user of this
@@ -637,25 +623,22 @@ ExecInitPartitionInfo(ModifyTableState *mtstate,
 					List	   *clause;
 
 					clause = copyObject((List *) node->onConflictWhere);
-					if (!equalTupdescs)
-					{
-						clause = (List *)
-							map_variable_attnos((Node *) clause,
-												INNER_VAR, 0,
-												part_attnos,
-												RelationGetDescr(firstResultRel)->natts,
-												RelationGetForm(partrel)->reltype,
-												&found_whole_row);
-						/* We ignore the value of found_whole_row. */
-						clause = (List *)
-							map_variable_attnos((Node *) clause,
-												firstVarno, 0,
-												part_attnos,
-												RelationGetDescr(firstResultRel)->natts,
-												RelationGetForm(partrel)->reltype,
-												&found_whole_row);
-						/* We ignore the value of found_whole_row. */
-					}
+					clause = (List *)
+						map_variable_attnos((Node *) clause,
+											INNER_VAR, 0,
+											part_attnos,
+											RelationGetDescr(firstResultRel)->natts,
+											RelationGetForm(partrel)->reltype,
+											&found_whole_row);
+					/* We ignore the value of found_whole_row. */
+					clause = (List *)
+						map_variable_attnos((Node *) clause,
+											firstVarno, 0,
+											part_attnos,
+											RelationGetDescr(firstResultRel)->natts,
+											RelationGetForm(partrel)->reltype,
+											&found_whole_row);
+					/* We ignore the value of found_whole_row. */
 					leaf_part_rri->ri_onConflict->oc_WhereClause =
 						ExecInitQual((List *) clause, &mtstate->ps);
 				}