Add check for matching column collations in ALTER TABLE ... INHERIT.

The other DDL operations that create an inheritance relationship were
checking for collation match already, but this one got missed.

Also fix comments that failed to mention collation checks.

src/backend/commands/tablecmds.c

@@ -1434,7 +1434,7 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
 
 				/*
 				 * Yes, try to merge the two column definitions. They must
-				 * have the same type and typmod.
+				 * have the same type, typmod, and collation.
 				 */
 				ereport(NOTICE,
 						(errmsg("merging multiple inherited definitions of column \"%s\"",
@@ -1620,7 +1620,7 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
 
 				/*
 				 * Yes, try to merge the two column definitions. They must
-				 * have the same type and typmod.
+				 * have the same type, typmod, and collation.
 				 */
 				ereport(NOTICE,
 				   (errmsg("merging column \"%s\" with inherited definition",
@@ -4121,7 +4121,7 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			int32		ctypmod;
 			Oid			ccollid;
 
-			/* Child column must match by type */
+			/* Child column must match on type, typmod, and collation */
 			typenameTypeIdAndMod(NULL, colDef->typeName, &ctypeId, &ctypmod);
 			if (ctypeId != childatt->atttypid ||
 				ctypmod != childatt->atttypmod)
@@ -8178,7 +8178,7 @@ MergeAttributesIntoExisting(Relation child_rel, Relation parent_rel)
 										  attributeName);
 		if (HeapTupleIsValid(tuple))
 		{
-			/* Check they are same type and typmod */
+			/* Check they are same type, typmod, and collation */
 			Form_pg_attribute childatt = (Form_pg_attribute) GETSTRUCT(tuple);
 
 			if (attribute->atttypid != childatt->atttypid ||
@@ -8189,6 +8189,17 @@ MergeAttributesIntoExisting(Relation child_rel, Relation parent_rel)
 								RelationGetRelationName(child_rel),
 								attributeName)));
 
+			if (attribute->attcollation != childatt->attcollation)
+				ereport(ERROR,
+						(errcode(ERRCODE_COLLATION_MISMATCH),
+						 errmsg("child table \"%s\" has different collation for column \"%s\"",
+								RelationGetRelationName(child_rel),
+								attributeName)));
+
+			/*
+			 * Check child doesn't discard NOT NULL property.  (Other
+			 * constraints are checked elsewhere.)
+			 */
 			if (attribute->attnotnull && !childatt->attnotnull)
 				ereport(ERROR,
 						(errcode(ERRCODE_DATATYPE_MISMATCH),