Why lost all foreign key constraits after cluster?

Why lost all foreign key constraits after cluster?

After I performed

cluster indexname on tablename;

All the foreign key constraints are lost on that table, why is that?
I get messages like

NOTICE: DROP TABLE implicitly drops referential integrity
trigger from table "tablename"

during cluster

I don't want to lost the foreign key constraints on the table. How do I
prevent
that? Thank you!


--Raymond