Extended relations

A description is given of extended relations, a generalization of ordinary relations in which tuple identifiers (tids) can appear as attribute values. The relational algebra is extended to include new operators that manipulate these tids; the resulting algebra is called the tid-algebra. The theorems of the tid-algebra are shown to provide a formal basis for optimizer transformations that involve high-level access structures. Three particular access structures-secondary indices, join indices, and join extensions-are examined in detail