P. Seshadri, H. Pirahesh, and T. Leung. Complex query decorrelation. In Proc. IEEE Conference on Data Engineering, pages 450--458, 1996.  Home/Search   Document Not in Database   Summary   Related Articles   Check

....the query plan ahead of the user while waiting for the next DOM traversal step, we do not consider such speculative execution in this paper. 4 The Decorrelation Plan Space Decorrelation has been studied in the context of generating equivalent executions for correlated SQL queries in [6, 11, 15, 24]. In all previous work of which we are aware, plans are decorrelated when possible on the heuristic assumption that the decorrelated execution can be optimized better. On the contrary in ROLEX, when the navigation profile indicates that a node will seldom be visited, correlated execution may be ....

P. Seshadri, H. Pirahesh, and T. Y. C. Leung. Complex query decorrelation. In Proc. of the 12th International Conference on Data Engineering, 1996.

....the query plan ahead of the user while waiting for the next DOM traversal step, we do not consider such speculative execution in this paper. 4 The Decorrelation Plan Space Decorrelation has been studied in the context of generating equivalent executions for correlated SQL queries in [6, 11, 15, 24]. In all previous work of which we are aware, plans are decorrelated when possible on the heuristic assumption that the decorrelated execution can be optimized better. On the contrary in ROLEX, when the navigation profile indicates that a node will seldom be visited, correlated execution may be ....

P. Seshadri, H. Pirahesh, and T. Y. C. Leung. Complex query decorrelation. In Proc. of the 12th International Conference on Data Engineering, 1996.

....Decorrelation In Section 4, we showed how complex expressions in XQuery are represented using correlations. However, it has been shown in earlier work that executing XML queries as correlated queries over a relational database leads to poor performance [11] We thus present query decorrelation [13] as a necessary step for efficient XML query execution. FUNCTION COMPOSES WITH REDUCTION 1 getTagName cr8Elem(Tag, Atts, Clist) Tag 2 getAttributes cr8Elem(Tag, Atts, Clist) Atts 3 getContents cr8Elem(Tag, Atts, Clist) Clist 4 getAttName cr8Att(Name, Val) Name 5 getAttValue cr8Att(Name, Val) ....

P. Seshadri, H. Pirahesh, C. Leung, "Complex Query Decorrelation", ICDE Conf., New Orleans, Louisiana, March 1996, pp. 450-458.

....sets of time instants. This translation correctly preserves the meaning of the original query. It is also easy to see that we cannot use optimization based on rewrites purely within SQL TP and therefore (modified) techniques for distinct elimination used by standard SQL optimizers, e.g. [5, 6, 8], cannot be used. In particular the (invalid) SQL TP expression SELECTa 1 ASa 1 , a k ASa k , t 1 AS t 1 , t l AS t l Q does not achieve our goal. Moreover, there is no SQL TP expression that could replace the original one in a duplicateinsensitive context that would simplify the ....

P. Seshadri, H. Pirahesh, and T. Y. C. Leung. Complex Query Decorrelation. In 12th International Conference on Data Engineering, pages 450--458, 1996.

....caching and common subexpressions; those approaches can coexist profitably with the techniques presented here. Correlated SQL subqueries can be considered as a form of expensive method [HS93] It has been demonstrated that the magic sets rewriting can be used to speed up such subqueries [MFPR90, SPL96] even in non recursive SQL. This magic decorrelation avoids redundant computation by first computing the (duplicate free) set of all input values to a correlated subquery, then feeding all the distinct input values into the subquery at once, and finally joining the result of all the subquery ....

Praveen Seshadri, Hamid Pirahesh, and T.Y. Cliff Leung. Complex Query Decorrelation. In Proc. 12th IEEE InternationalConference on Data Engineering, New Orleans, February 1996.

....to the lhs and rhs of a rewrite rule) written in C. Because they are expressed in C, Starburst s query rewriting rules are able to express a wide variety of transformations including view merging, nested query unnesting (both discussed in [PHH92] and magic sets transformations ( MFPR90, SPL96] However, Starburst rules are difficult to understand and reason about, requiring a detailed understanding of the underlying graph based query representation (QGM) Exodus [CDG 90] and its successors, Volcano [GM93] and Cascades [Gra95] and ESL [FG94] use rules that resemble rewrite ....

....benefits, speeding up the process of query rewriting and freeing up more time for an optimizer to perform its search. Still to write about: ffl Opt [KD] 7. 3 Transforming Nested Queries Still to write about: ffl Kim [Kim82] ffl Ganski Wong [GW87] ffl Dayal [Day87] ffl Magic Sets [MP92, SPL96, SHP 96b] ffl Cluet and Moerkotte [CM95, CM93] ffl Steenhagen [SABd94] 7.4 Semantic Optimization and Handling Foreign Functions ffl Zdonik [HZ80] ffl Aberer [AF95] ffl Chaudhuri and Kim [CS96, CS93] ffl Conjunctive Predicates [HS93, Hel94, LMS94] ffl Avoiding sorting [SSM96] ffl ....

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Praveen Seshadri, Hamid Pirahesh, and T.Y. Cliff Leung. Complex query decorrelation. In Stanley Y. W. Su, editor, Twelfth International Conference on Data Engineering, pages 450--458, New Orleans, LA, February 1996. IEEE, IEEE Computer Science Press.

.... many proposals have utilized functional dependencies and foreign key constraints to optimize queries, e.g. for predicate movement [16] as a supplement to Magic Set optimization for bag semantics [18] for optimization of joins and semi joins [19] or for decorelation of complex queries [22], and other more general integrity constraints to optimize queries [12] This paper links the e orts seeking ecient decision procedures for dialects of description logics [4] with actual techniques used for optimizing queries. The link between these two techniques is non trivial: conjunctive ....

Praveen Seshadri, Hamid Pirahesh, and T. Y. Cli Leung. Complex Query Decorrelation. In 12th International Conference on Data Engineering, pages 450-458, 1996.

....other cases. A natural proposal is to apply it to aggregation operators which are known to be delicate to analyze, such as count [Kim82, GW87, Mur92] Finally, it should be possible to extend our approach to cover SQL queries with the all quantifier; and incorporate other optimization algorithms [RR98, SPL96] within our general framework. ....

P. Seshadri, H. Pirahesh, T. Y. C. Leung. Complex Query Decorrelation. In ICDE 96.

....support queries, despite of their importance. To express such queries in SQL, a high degree of redundancy is required: multiple self joins, correlated subqueries and repeated group bys. This leads to complicated queries, difficult to understand and optimize. Standard query processing techniques [8, 22] help somehow. The problem is that a traditional SQL optimizer will not consider the big picture , but will try to optimize a series of joins and aggregations, which is not always the best approach. In [7] we have addressed this issue and provided techniques to combine joins and aggregations into ....

P. Seshadri, H. Pirahesh, and T. C. Leung. Complex Query Decorrelation. In International Conference on Data Engineering, pages 450--458, 1996.

....has to be executed from scratch multiple times, this strategy can be very time consuming. To overcome this problem, query decorrelation has been studied and proposed as a better solution. The basic idea is to rewrite a correlated query in such a way that outer references no longer exist. In [SPL96] the authors proposed a technique that will extract all the distinct values of outer references and materialize all the possible results from the subquery. Later, the materialized results are joined with the outer query block on the outer reference values. Although the rewritten query usually ....

....C (a) Plan 1 (b) Plan 2 Figure 1: Plans for a correlated subquery out in [PHH92] queries that contain non existential or non Boolean factor subqueries, set operators, aggregates, or user defined extension operators (e.g. OUTER JOIN) do not get rewritten. Although it is shown in later work ( SPL96] that some of the queries including aggregates can be decorrelated, there are still many queries which we don t want to rewrite for either semantic or implementation reasons. For example, correlated queries with nested predicate using a noninclusion operator (NOT IN) are almost impossible to ....

[Article contains additional citation context not shown here]

Praveen Seshadri, Hamid Pirahesh, and T.Y.Cliff Leung. Complex query decorrelation. In Proc. IEEE Int'l Conf. on Data Eng., pages 450--458, 1996.

....that every group query can be expressed in a form that satisfies our criterion. 2.1 Assumptions and Terminology We assume that queries are written in terms of views, with no subqueries. This is a valid assumption since there are many ways to rewrite a subquery as a join of two (or more) views [Kim82, Day87, SPL96]. We initially assume that the database contains a single relation R. Multiple relations will be considered in Section 2.4. R may itself be a view or the result of another query, but from our point of view it is treated as an encapsulated table. i.e. if R was a view, then we don t consider ....

....context of distributed query processing and horizontally partitioned relations [SA80, ESW78, Day83] In these papers, one of the key ideas for query optimization is that selection, projection and join distribute over union, a key idea also in our work. Kim in [Kim82] Dayal in [Day87] Seshadri in [SPL96] propose decorrelation techniques, where an SQL correlated query is transformed to a query that is a join of base tables and one or more aggregate views. When rewritten using views, the attributes used for equality correlations tend to become partitioning attributes in our sense. Thus our ....

Praveen Seshadri, Hamid Pirahesh, and T.Y. Cliff Leung. Complex query decorrelation. In International Conference of Data Engineering, pages 450--458, 1996.

....depending on the complexity and size of the subquery. While some subquery predicates can be converted into joins (thereby becoming subject to traditional join based optimization and execution strategies) even sophisticated SQL rewrite systems such as that of DB2 CS [Pirahesh et al. 1992; Seshadri et al. 1996; Seshadri et al. 1996] cannot convert all subqueries to joins. When one is forced to compute a subquery in order to evaluate a predicate, then the predicate should be treated as an expensive method. Thus the work presented in this paper is applicable to the majority of today s production RDBMSs, ....

....the complexity and size of the subquery. While some subquery predicates can be converted into joins (thereby becoming subject to traditional join based optimization and execution strategies) even sophisticated SQL rewrite systems such as that of DB2 CS [Pirahesh et al. 1992; Seshadri et al. 1996; Seshadri et al. 1996] cannot convert all subqueries to joins. When one is forced to compute a subquery in order to evaluate a predicate, then the predicate should be treated as an expensive method. Thus the work presented in this paper is applicable to the majority of today s production RDBMSs, which support SQL ....

Seshadri, P., Pirahesh, H., and Leung, T. C. 1996. Complex Query Decorrelation. In Proc. 12th IEEE International Conference on Data Engineering (New Orleans, Feb. 1996).

....repeated creation and copying of CLOBs can be costly. 4.1.3. The Decorrelated CLOB Approach One disadvantage of the Correlated CLOB approach is that, because of its correlated subqueries, it naturally implies a nested loop join strategy. This can be avoided by performing query de correlation [21] inside the relational engine to give the relational optimizer more flexibility. The query execution plan obtained by de correlating the query in Figure 3 is shown in Figure 7. Figure 6: SQL Query Execution Plan for the Correlated CLOB Approach Customer CUST( custXML) custId, custName) ....

P. Seshadri, H. Pirahesh, T. Y. C. Leung, "Complex Query Decorrelation", Proceedings of the International Conference on Data Engineering (ICDE), Louisiana, USA, February 1996.

....repeated creation and copying of CLOBs can be costly. 4.1.3 The De Correlated CLOB Approach One disadvantage of the Correlated CLOB approach is that, because of its correlated sub queries, it naturally implies a nested loop join strategy. This can be avoided by performing query de correlation [12] inside the relational engine to give the relational optimizer more flexibility. A de correlated query execution plan for the correlated query of Figure 3 is shown in Figure 6. Though the Item and Payment tables are ignored for clarity, it is easy to see how this approach generalizes to arbitrary ....

P. Seshadri, H. Pirahesh, T. Y. C. Leung, "Complex Query Decorrelation," International Conference on Data Engineering (ICDE), Louisiana, February 1996.

....what physical properties they should have. Index selection then falls out as simply a special case of choosing physical properties, with absolutely no changes to our algorithms. Next we consider nested queries. One approach to handling nested queries is to use decorrelation techniques (see, e.g. SPL96] The use of such decorrelation techniques results in the query being transformed 3 This cost heap is not to be confused with the heap on topological numbering used earlier. 12 to a set of queries, with temporary relations being created. Now, the queries generated by decorrelation have ....

Praveen Seshadri, Hamid Pirahesh, and T. Y. Cliff Leung. Complex query decorrelation. In Intl. Conf. on Data Engineering, 1996.

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P. Seshadri, H. Pirahesh, and T. Leung. Complex query decorrelation. In Proc. IEEE Conference on Data Engineering, pages 450--458, 1996.

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P. Seshadri, H. Pirahesh, and T. Leung. Complex query decorrelation. In Proc. IEEE Conference on Data Engineering, pages 450--458, 1996.

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P. Seshadri, H. Pirahesh, and T. C. Leung. Complex Query Decorrelation. In ICDE, 1996.

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P. Seshadri, H. Pirahesh, and T. Leung. Complex query decorrelation. In Proc. IEEE Conference on Data Engineering, pages 450--458, 1996.

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P. Seshadri, H. Pirahesh, and T. Leung. Complex query decorrelation. In Proc. IEEE Conference on Data Engineering, pages 450--458, 1996.

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P. Seshadri, H. Pirahesh, and T. Y. Cli# Leung. Complex query decorrelation. In ICDE, 1996.

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P. Seshadri, H. Pirahesh, and T. Leung. Complex query decorrelation. In Proc. IEEE Conference on Data Engineering, pages 450--458, 1996.

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Praveen Seshadri, Hamid Pirahesh, and T. Y. Cliff Leung. Complex query decorrelation. In Proceedings of the Twelfth International Conference on 183 Data Engineering, February 26 - March 1, 1996, New Orleans, Louisiana, pages 450--458, 1996.

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Seshadri, P., Pirahesh, H., & Cliff Leung, T. Y., Complex query decorrelation. In Proceedings International Conference on Data Engineering 1996, pages 450-458. IEEE Computer Society, Los Alamitos, CA, March 1996.

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Praveen Seshadri, Hamid Pirahesh, and T.Y. Cliff Leung. Complex Query Decorrelation. In Proceedings 12 th IEEE Conference on Data Engineering, New Orleans, February, 1996.

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