Oracle Core: Essential Internals for DBAs and Developers

Data Block Visits and Undo

Any time your session looks at a data block, it needs to ensure that what you see is the appropriate version of the data. This means that, from an external point of view, your session should not see any uncommitted data, or data that was modified and committed since the start of your query (or DML statement or even transaction—depending on the isolation level). This is referred to as a read-consistent version of the data.

Note It’s easy to forget that read consistency is also a necessary prerequisite to changing data. If your session is supposed to modify the data in a block, then, from an internal point of view, it ...