make ALTER DOMAIN VALIDATE CONSTRAINT no-op when constraint is validated

Looking at AlterDomainValidateConstraint, it seems currently, ALTER DOMAIN
VALIDATE CONSTRAINT will re-validate a constraint that has already been
validated, which would just waste cycles.  Ideally, this operation should be a
no-op when the constraint is already validated.

Author: jianhe-fun

src/backend/commands/typecmds.c

@@ -3087,24 +3087,29 @@ AlterDomainValidateConstraint(List *names, const char *constrName)
 				 errmsg("constraint \"%s\" of domain \"%s\" is not a check constraint",
 						constrName, TypeNameToString(typename))));
 
-	val = SysCacheGetAttrNotNull(CONSTROID, tuple, Anum_pg_constraint_conbin);
-	conbin = TextDatumGetCString(val);
+	if (!con->convalidated)
+	{
+		val = SysCacheGetAttrNotNull(CONSTROID, tuple, Anum_pg_constraint_conbin);
+		conbin = TextDatumGetCString(val);
 
-	validateDomainCheckConstraint(domainoid, conbin);
+		validateDomainCheckConstraint(domainoid, conbin);
 
-	/*
-	 * Now update the catalog, while we have the door open.
-	 */
-	copyTuple = heap_copytuple(tuple);
-	copy_con = (Form_pg_constraint) GETSTRUCT(copyTuple);
-	copy_con->convalidated = true;
-	CatalogTupleUpdate(conrel, &copyTuple->t_self, copyTuple);
+		/*
+		 * Now update the catalog, while we have the door open.
+		*/
+		copyTuple = heap_copytuple(tuple);
+		copy_con = (Form_pg_constraint) GETSTRUCT(copyTuple);
+		copy_con->convalidated = true;
+		CatalogTupleUpdate(conrel, &copyTuple->t_self, copyTuple);
 
-	InvokeObjectPostAlterHook(ConstraintRelationId, con->oid, 0);
+		InvokeObjectPostAlterHook(ConstraintRelationId, con->oid, 0);
 
-	ObjectAddressSet(address, TypeRelationId, domainoid);
+		ObjectAddressSet(address, TypeRelationId, domainoid);
 
-	heap_freetuple(copyTuple);
+		heap_freetuple(copyTuple);
+	}
+	else
+		address = InvalidObjectAddress; /* already validated */
 
 	systable_endscan(scan);