> Does the indexes created with FK constraints(in T2) with many repeted values
> will cause some problems such as poor plan or in selectivity of these
> indexes in some situations? Will have other effects?

Luiz

I don't think there's a valid answer for this.

You've got to find a balance between the poor selectivity (if your field
C2 hasn't got much possible values) and the "security" that a
fk-constraint does offer.

You can check the plan for the queries you use and if you think it's a
poor one you still got the choice between using a [LEFT] JOIN and an
inline select.

Try to figure this out with tools like (for example) IBExpert or
QuickDesk to get the performance statistics.

HTH

Luc.