The reason for 2x fetches is that Firebird identifies a record through a slot in a vector on the data page - both the page and the slot are part of the RDB$DB_KEY.   The page is identified indirectly through an index of pages for the table.  The values in the vector on the data page are the length of the stored record and the offset on the page of the first byte of the record.  

 Going through a record locator vector means that records can move within a page without affecting

anything off the page, so it's very cheap to repack a page to reclaim fragments of space.

But you already figured that out yourselves....

Cheers,

Ann