>> There are no trigger, it is checked by the corresponding index

> I don't understand. There is a unique index on the parent's columns,
> and a non-unique index on the child's columns. Something must run to
> confirm that the value added/updated in the child exists in the parent's
> index.

Sure, but that is part of the engine code, not some kind of a trigger (IIRC,
system triggers are used for CASCADE option in FK).

If I understand correctly, this discussion goes more or less in the
direction of deferred constraint checks, though in that case check is
delayed until the end of transaction, but not deactivated completely. But
that would be a discussion for Firebird-Architect list (IIRC, there was one
already, but nobody had any usable idea).

Roman