> I would expect that Firebird only read 10 record, however it reads 10 +
> 10x20 = 210 records.

> > In theory the optimizer could know that City.ID and Hobby.ID  are both
> primary keys (or unique) and therefore only scan PersonTable.

The engine/optimizer does know that.

The engine, however, must check whether the matching key does exist, which requires reading the index structures. -- which raises a READ or FETCH operation!

So, a count of 210 is not unreasonable.


Sean