Glossary

?: Same as functionally determines (functional dependencies are introduced in Chapter 4). A ? B if any two rows having the same A value also have the same B value.
2-3 tree: B-tree whose maximum branching factor is 3 and minimum branching factor is 2. A tree with a small branching factor such as a 2-3 tree or a T-tree should be used for small data structures that remain in random access memory (Chapter 3).
4GL: Same as fourth-generation language (Chapter 5). These languages often generate very poor SQL.

abort: Every database transaction completes either by committing or aborting. When a transaction aborts, all its writes are undone by the recovery subsystem. A transaction abort is often caused by a deadlock (Chapter 2).
access path: Strategy used to access the rows of a table. The most common alternatives are a direct access (i.e., a table scan) or an access through an index (i.e., an index scan) (Chapter 7 and Appendix D).
after image: The after image of a data item x with respect to transaction T is the value of x that T last writes (Chapter 2).
aggregate maintenance: Maintenance of one or more redundant relations that embody an aggregate. For example, if the sum of sales by store is frequently requested, then it may be worthwhile to maintain a relation containing that sum and then to update it whenever a sale occurs (Chapters 4 and 10).
aggregate targeting: A...

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Database Tuning: Principles, Experiments, and Troubleshooting Techniques