A sentence or two went AWOL.

>
> Eventually, when the database engine realises there is no transaction
> that could be used

Eventually, when the database engine realises there is no transaction
that could be interested, it can be deleted and the space it occupied
can be reused. Whether this occurs through co-operation or via a
separate thread or both depends on your Firebird version and whether
it is classic or superserver architecture (and in some versions what
is in your firebird.conf file).

The point is that a record version is not held indefinitely, but it
may be held longer than is immediately obvious, and that cleanup can
be expensive before Firebird 2 is your index has lots of dups.

Adam