Add TupleDescGetDefault()

This unifies some repetitive code.

Note: I didn't push the "not found" error message into the new
function, even though all existing callers would be able to make use
of it.  Using the existing error handling as-is would probably require
exposing the Relation type via tupdesc.h, which doesn't seem
desirable.  (Or even if we changed it to just report the OID, it would
inject the concept of a relation containing the tuple descriptor into
tupdesc.h, which might be a layering violation.  Perhaps some further
improvements could be considered here separately.)

Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da%40eisentraut.org

src/backend/access/common/tupdesc.c

@@ -927,3 +927,28 @@ BuildDescFromLists(const List *names, const List *types, const List *typmods, co
 
 	return desc;
 }
+
+/*
+ * Get default expression (or NULL if none) for the given attribute number.
+ */
+Node *
+TupleDescGetDefault(TupleDesc tupdesc, AttrNumber attnum)
+{
+	Node	   *result = NULL;
+
+	if (tupdesc->constr)
+	{
+		AttrDefault *attrdef = tupdesc->constr->defval;
+
+		for (int i = 0; i < tupdesc->constr->num_defval; i++)
+		{
+			if (attrdef[i].adnum == attnum)
+			{
+				result = stringToNode(attrdef[i].adbin);
+				break;
+			}
+		}
+	}
+
+	return result;
+}

src/backend/commands/tablecmds.c

@@ -2795,22 +2795,9 @@ MergeAttributes(List *columns, const List *supers, char relpersistence,
 			 */
 			if (attribute->atthasdef)
 			{
-				Node	   *this_default = NULL;
+				Node	   *this_default;
 
-				/* Find default in constraint structure */
-				if (constr != NULL)
-				{
-					AttrDefault *attrdef = constr->defval;
-
-					for (int i = 0; i < constr->num_defval; i++)
-					{
-						if (attrdef[i].adnum == parent_attno)
-						{
-							this_default = stringToNode(attrdef[i].adbin);
-							break;
-						}
-					}
-				}
+				this_default = TupleDescGetDefault(tupleDesc, parent_attno);
 				if (this_default == NULL)
 					elog(ERROR, "default expression not found for attribute %d of relation \"%s\"",
 						 parent_attno, RelationGetRelationName(relation));

src/backend/parser/parse_utilcmd.c

@@ -1358,20 +1358,11 @@ expandTableLikeClause(RangeVar *heapRel, TableLikeClause *table_like_clause)
 				 (table_like_clause->options & CREATE_TABLE_LIKE_GENERATED) :
 				 (table_like_clause->options & CREATE_TABLE_LIKE_DEFAULTS)))
 			{
-				Node	   *this_default = NULL;
-				AttrDefault *attrdef = constr->defval;
+				Node	   *this_default;
 				AlterTableCmd *atsubcmd;
 				bool		found_whole_row;
 
-				/* Find default in constraint structure */
-				for (int i = 0; i < constr->num_defval; i++)
-				{
-					if (attrdef[i].adnum == parent_attno)
-					{
-						this_default = stringToNode(attrdef[i].adbin);
-						break;
-					}
-				}
+				this_default = TupleDescGetDefault(tupleDesc, parent_attno);
 				if (this_default == NULL)
 					elog(ERROR, "default expression not found for attribute %d of relation \"%s\"",
 						 parent_attno, RelationGetRelationName(relation));

src/backend/rewrite/rewriteHandler.c

@@ -1246,21 +1246,7 @@ build_column_default(Relation rel, int attrno)
 	 */
 	if (att_tup->atthasdef)
 	{
-		if (rd_att->constr && rd_att->constr->num_defval > 0)
-		{
-			AttrDefault *defval = rd_att->constr->defval;
-			int			ndef = rd_att->constr->num_defval;
-
-			while (--ndef >= 0)
-			{
-				if (attrno == defval[ndef].adnum)
-				{
-					/* Found it, convert string representation to node tree. */
-					expr = stringToNode(defval[ndef].adbin);
-					break;
-				}
-			}
-		}
+		expr = TupleDescGetDefault(rd_att, attrno);
 		if (expr == NULL)
 			elog(ERROR, "default expression not found for attribute %d of relation \"%s\"",
 				 attrno, RelationGetRelationName(rel));

src/include/access/tupdesc.h

@@ -151,4 +151,6 @@ extern TupleDesc BuildDescForRelation(const List *columns);
 
 extern TupleDesc BuildDescFromLists(const List *names, const List *types, const List *typmods, const List *collations);
 
+extern Node *TupleDescGetDefault(TupleDesc tupdesc, AttrNumber attnum);
+
 #endif							/* TUPDESC_H */