Subject Re: [firebird-support] how read commited is implemented
Author Ann Harrison
Note that read-committed and repeatable read are different modes.
Read-committed, by definition, is not repeatable. For your other
questions, search the IBPhoenix site, especially the list archives,
for information on versioning. There's a lot there and I'm disinclined
to repeat it all here.

I will say, however, that the newest version of a record maintains the
record identifier,
back versions are chained, stored on page and, where possible, reduced
to deltas
(i.e. the amount of information necessary to reconstruct the older
version). They
are removed when no active transaction requires them.