With schema versioning, each table has a version number that identifies the current logical record layout. Each record in the database has a version number that defines its physical layout.
When you make a schema change, a table's schema is updated and its version number changes. The records stored in the database are not changed during the schema change.
Later when a record is fetched, it is converted to the new version (if it is using the old version) before it is returned from the storage manager. If the record is subsequently updated, it is written back to the database with the updated layout and version.
As a result, the writing of the converted record to disk is now combined with the original write operation of a record. Thus, you're getting two operations performed where one write operation had been previously performed. This technique makes it possible to update the schema without the need to go through each of the records again when the actual schema change occurs. In addition, the risk of destroying the before image log is also eliminated. |
