Function ATExecAddConstraint

src/backend/commands/tablecmds.c:10706–10774

* ALTER TABLE ADD CONSTRAINT * * Return value is the address of the new constraint; if no constraint was * added, InvalidObjectAddress is returned. */

 * added, InvalidObjectAddress is returned.
 */
static ObjectAddress
ATExecAddConstraint(List **wqueue, AlteredTableInfo *tab, Relation rel,
					Constraint *newConstraint, bool recurse, bool is_readd,
					LOCKMODE lockmode)
{
	ObjectAddress address = InvalidObjectAddress;

	Assert(IsA(newConstraint, Constraint));

	/*
	 * Currently, we only expect to see CONSTR_CHECK and CONSTR_FOREIGN nodes
	 * arriving here (see the preprocessing done in parse_utilcmd.c).  Use a
	 * switch anyway to make it easier to add more code later.
	 */
	switch (newConstraint->contype)
	{
		case CONSTR_CHECK:
			address =
				ATAddCheckConstraint(wqueue, tab, rel,
									 newConstraint, recurse, false, is_readd,
									 lockmode);
			break;

		case CONSTR_FOREIGN:

			/*
			 * Assign or validate constraint name
			 */
			if (newConstraint->conname)
			{
				if (ConstraintNameIsUsed(CONSTRAINT_RELATION,
										 RelationGetRelid(rel),
										 newConstraint->conname))
					ereport(ERROR,
							(errcode(ERRCODE_DUPLICATE_OBJECT),
							 errmsg("constraint \"%s\" for relation \"%s\" already exists",
									newConstraint->conname,
									RelationGetRelationName(rel))));
			}
			else
				newConstraint->conname =
					ChooseConstraintName(RelationGetRelationName(rel),
										 ChooseForeignKeyConstraintNameAddition(newConstraint->fk_attrs),
										 "fkey",
										 RelationGetNamespace(rel),
										 NIL);

			address = ATAddForeignKeyConstraint(wqueue, tab, rel,
												newConstraint,
												recurse, false,
												lockmode);
			break;

		default:
			elog(ERROR, "unrecognized constraint type: %d",
				 (int) newConstraint->contype);
	}