Function ATExecDropConstraint

src/backend/commands/tablecmds.c:13469–13728

* ALTER TABLE DROP CONSTRAINT * * Like DROP COLUMN, we can't use the normal ALTER TABLE recursion mechanism. */

 * Like DROP COLUMN, we can't use the normal ALTER TABLE recursion mechanism.
 */
static void
ATExecDropConstraint(Relation rel, const char *constrName,
					 DropBehavior behavior,
					 bool recurse, bool recursing,
					 bool missing_ok, LOCKMODE lockmode)
{
	List	   *children;
	ListCell   *child;
	Relation	conrel;
	Form_pg_constraint con;
	SysScanDesc scan;
	ScanKeyData skey[3];
	HeapTuple	tuple;
	bool		found = false;
	bool		is_no_inherit_constraint = false;
	char		contype;

	/* At top level, permission check was done in ATPrepCmd, else do it */
	if (recursing)
		ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);

	conrel = table_open(ConstraintRelationId, RowExclusiveLock);

	/*
	 * Find and drop the target constraint
	 */
	ScanKeyInit(&skey[0],
				Anum_pg_constraint_conrelid,
				BTEqualStrategyNumber, F_OIDEQ,
				ObjectIdGetDatum(RelationGetRelid(rel)));
	ScanKeyInit(&skey[1],
				Anum_pg_constraint_contypid,
				BTEqualStrategyNumber, F_OIDEQ,
				ObjectIdGetDatum(InvalidOid));
	ScanKeyInit(&skey[2],
				Anum_pg_constraint_conname,
				BTEqualStrategyNumber, F_NAMEEQ,
				CStringGetDatum(constrName));
	scan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId,
							  true, NULL, 3, skey);

	/* There can be at most one matching row */
	if (HeapTupleIsValid(tuple = systable_getnext(scan)))
	{
		ObjectAddress conobj;

		con = (Form_pg_constraint) GETSTRUCT(tuple);

		/* Don't drop inherited constraints */
		if (con->coninhcount > 0 && !recursing)
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
					 errmsg("cannot drop inherited constraint \"%s\" of relation \"%s\"",
							constrName, RelationGetRelationName(rel))));

		is_no_inherit_constraint = con->connoinherit;
		contype = con->contype;