Nested queries and quantifiers in an ordered context

Abstract

We present algebraic equivalences that allow to unnest nested algebraic expressions for order-preserving algebraic operators. We illustrate how these equivalences can be applied successfully to unnest nested queries given in the XQuery language. Measurements illustrate the performance gains possible by unnesting.

Three Cases for Query Decorrelation in XQuery

“…SAL is the order-preserving counterpart of the algebra used in [5,6] extended to handle semistructured data. Other algebras have been proposed (see [22]), but we omit this discussion because this paper focuses on unnesting.…”

“…For space reasons we omit the details of the normalization and translation (see [22] for details). We concentrate on the equivalences used for unnesting our example queries.…”

“…[a: "author1", t: "title1"] [a: "author1", t: "title2"] [a: "author2", t: "title1"] [a: "author2", t: "title3"] Then s1 Ξ s3 a;s2 (e) with s1 = "<author>";"<name>";a;"</name>" s2 = "<title>";t;"</title>" s3 = "</author>"} produces <author> <name>author1</name> <title>title1</title> <title>title2</title> </author> <author> <name>author2</name> <title>title1</title> <title>title3</title> </author> For implementation issues concerning the operators of NAL, please consult our technical report [22].…”

“…All these techniques are described by means of algebraic equivalences which we proved to be correct in our technical report [22]. We also provide performance figures for every query execution plan demonstrating the significant speed-up gained by unnesting.…”

“…Our Unnesting Approach consists of three steps (in this paper, we focus on the third step, details on the first two steps can be found in [22]):…”