Garbage Collection

Garbage collection prevents an update-intensive database from filling up with unnecessary back versions of records. It also removes record versions created by transactions that rolled back. Every transaction participates in garbage collection - every transaction, including read-only transactions.

When a client applicaition reads a record from a Firebird database, it gets a record that looks like any record from any database. Two levels lower, somewhere in the server, InterBase/Firebird pulls a string of record versions off the disk. Each version is tagged with the transaction id of the transaction that created it. The first one is the most recently stored. At this point, the server has two goals: 1) produce an appropriate version of the record for the current transaction 2) remove any versions that are garbage - either because they were created by a transaction that rolled back or because they are so old that nobody will ever want to see them again.

Taken from IBPhoenix