Sybase IQ uses snapshot versioning to allow transactions to operate concurrently.
You can think of snapshot versioning as you would a snapshot you take with a camera. When you photograph a snapshot of an object or scene, you get an image of it as it appears at a given moment in time. Likewise, when IQ takes a snapshot of an object in your database, it retains an image of that object at a given instant in time.
Unlike a camera, though, IQ does not need to make a copy of the entire object each time the image changes. Instead, it copies only the parts of the image—the database pages—that have changed. Database pages that have not changed are shared among all active versions in the database.
IQ takes its snapshot when the first command is executed following a connect, commit, or rollback. The user can force the snapshot to be taken earlier by executing an explicit BEGIN TRANSACTION command. Throughout the transaction, a user who reads from the object sees the unchanged image, or snapshot version.
