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

1 files changed, 2 insertions, 15 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index ad398e837d5..73b8dea81c8 100644
--- a/src/backend/commands/tablecmds.c
+++ b/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));