R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.  Home/Search   Document Not in Database   Summary   Related Articles   Check

....as one evaluation technique) except that tree scheme occurrences are used as a representation of (intermediate) results. Later, Day87] proposed the G Join, G Aggr and G Restr. The G Join replaces the graft operator and a sequence of G Aggr and G Restr replaces the previously used prune operator. [GL87] treated queries with quantification as a special case of nested queries. The quantifiers exists and not exists are replaced by count aggregations. More recently, Steenhagen [Ste95] investigated rules for unnesting queries in an object oriented model. In this paper we begin with a systematic ....

R. A. Ganski and H. K. T. Long. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 22--33, San Francisco, USA, May 1987.

....problem. Even in the relational case, very complex rule structures are reported [18] The structures which have to be taken into account in our model are much more complex than those in the relational case. Hence, the language for rules used to deal with these structures also becomes more complex [14]. 3.3 The Execution of Statements The QEP of a query is an operator graph that describes an evaluation sequence at quite an abstract level. During the execution phase, the operator graph is interpreted node by node. Together with the data of the specific database it is used to compute the ....

R.A. Ganski and H.K.T. Wong,, Optimization of Nested SQL Queries Revisited, in: Proc. ACM SIGMOD Int. Conf. on Management of Data, San Francisco (1987) 23-33.

....queries to concrete evaluation algorithms. We are presenting a new XML algebra and a query optimization framework based on query normalization and query unnesting (also known as query decorrelation) There are many proposals on query optimization that are focused on unnesting nested queries [23, 18, 26, 11, 12, 9, 28]. Nested queries appear more often in XML queries than in relational queries, because most XML query languages, including XQuery, allow complex expressions at any point in a query. Current commercial database systems typically evaluate nested queries in a nested loop fashion, which is unacceptable ....

R. Ganski and H. Wong. Optimization of Nested SQL Queries Revisited. In Proceedings of the ACMSIGMOD International Conference on Management of Data, San Francisco, California, pages 23-33, May 1987.

.... been noted that the nested form is often easier for users to formulate and to understand [12] Existing research (on nested queries) has sought methods of reducing the evaluation costs, typically by transforming a nested query into a logically equivalent form that can be evaluated more efficiently [2, 3, 10]. However, the class of nested queries with aggregates is especially interesting. First, such queries are commonly encountered. For example, to list employees who are high earners where highearners are defined as a certain factor of the average salaries of employees, requires a nested query ....

R. Ganski and H.K.T. Wong. Optimization of nested sql queries revisited. In SIGMOD'87, pages 23--33, June 1987.

....that computes join and antijoin simultaneously, we need a semijoin based strategy, in the general case. 5 Conclusions and comparison with related work Related work. After the definition given in [LP76] a number of papers have used outerjoins to formulate certain classes of queries, e.g. [DH84, Dat86b, Day87, GW87, Mur89, OMO89, WM90, Dav91]. But a key problem with outerjoins is their lack of associativity [Dat86b, Dat86a, Mai83] which makes more difficult the appropriate formulation of queries, as well as their evaluation. Papers dealing with evaluation of queries containing outerjoins include [RR84, Che90, RGL90, GLR92, GLR93, ....

R. A. Ganski and H. K. T. Wong. Optimization of nested sql queries revisited. In Proceedings of ACM-SIGMOD 1987 International Conference on Management of Data, San Francisco, pages 23--33, 1987.

....efficiently. However, there exist some major differences between nested queries and joins that are, as stated in [12] the creation of duplicates and the way empty tables are handled. Indeed, these differences are at the bottom of most of the bugs subsequently detected in the original algorithms [10, 17, 6, 8, 11, 12]. To solve them and among other techniques, 8, 11] introduced outer joins. While these algebraic operators are a nice solution to some of the above problems, they raise new issues. If a sequence of join operations can easily be reordered, joins and outer joins do not commute that easily and one ....

....nested queries and joins that are, as stated in [12] the creation of duplicates and the way empty tables are handled. Indeed, these differences are at the bottom of most of the bugs subsequently detected in the original algorithms [10, 17, 6, 8, 11, 12] To solve them and among other techniques, [8, 11] introduced outer joins. While these algebraic operators are a nice solution to some of the above problems, they raise new issues. If a sequence of join operations can easily be reordered, joins and outer joins do not commute that easily and one has to find new ways of dealing with multi layered ....

R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.

....sets of rules. This provides many performance 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, ....

Richard A. Ganski and Harry K. T. Wong. Optimization of nested SQL queries revisited. In Umeshwar Dayal and Irv Traiger, editors, Proceedings of the SIGMOD International Conference on Management of Data, pages 23--33, San Francisco, California, May 1987. ACM Special Interest Group on Management of Data, ACM Press.

....issue is given in Chapter 8 where the support of views is discussed. Nested queries are crucial in the construction of new objects especially complex objects. It has been shown that many nested queries which appear only in the where clause (the filters) can be eliminated from SQL queries [Kim82, GW87] In an object oriented query language supporting free nesting of queries, it it not obvious how nested queries can be eliminated without other language constructs such as query functions. Query nesting can also be considered as an issue of generality. Quantifiers can simplify queries and ....

R.A. Ganski and H.K.T. Wong. Optimization of Nested SQL Queries Revisited. Proceedings of the ACM SIGMOD International Conference on Management of Data, pages 23--33, 1987.

....which lead naturally to nested queries. Current OODB systems typically evaluate nested queries in a nested loop fashion, which does not leave many opportunities for optimization. Most unnesting techniques for OODB queries are actually based on similar techniques for relational queries [Kim 1982; Ganski and Wong 1987; Muralikrishna 1992] For all but the trivial nested queries, these techniques require the use of outer joins, to prevent loss of data, and grouping, to accumulate the data and to remove the null values introduced by the outer joins. 30 L. Fegaras and D. Maier If considered in isolation, query ....

....actually source to source transformations over SQL code, mostly due to the lack of a group by operator in the relational algebra to express grouping. The absence of a formal theory in a form of an algebra to express these transformations resulted in a number of bugs (such as the infamous count bug [Ganski and Wong 1987]) which were eventually detected and corrected. Since OODB queries are far more complex than relational queries, it is more crucial to express the unnesting transformations in a formal algebra that will allow us to prove the soundness and completeness of these transformations. The rst work with ....

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Ganski, R. and Wong, H. 1987. Optimization of Nested SQL Queries Revisited. In Proceedings of the ACM-SIGMOD International Conference on Management of Data, San Francisco, California (May 1987), pp. 23-33.

....which lead naturally to nested queries. Current OODB systems typically 26 evaluate nested queries in a nested loop fashion, which does not leave many opportunities for optimization. Most unnesting techniques for OODB queries are actually based on similar techniques for relational queries [Kim82, GW87, Mur92] For all but the trivial nested queries, these techniques require the use of outer joins, to prevent loss of data, and grouping, to accumulate the data and to remove the null values introduced by the outer joins. If considered in isolation, query unnesting itself does not result in ....

....source to source transformations over SQL code, mostly due to the lack of a group by operator in the relational algebra to express grouping. The absence of a formal theory in a form of an algebra to express these transformations resulted in a number of bugs (such as the infamous count bug [GW87] which were eventually detected and corrected. Since OODB queries are far more complex than relational queries, it is more crucial to express the unnesting transformations in a formal algebra that will allow us to prove the soundness and completeness of these transformations. The rst work with ....

[Article contains additional citation context not shown here]

R. Ganski and H. Wong. Optimization of Nested SQL Queries Revisited. In Proceedings of the ACMSIGMOD International Conference on Management of Data, San Francisco, California, pages 23-33, May 1987. 44

....take a more complicated nested query and produce two or more subqueries that compute the same result. Some subqueries are not flat, but their nesting patterns are simpler than the nesting pattern of the untransformed query. These subqueries can be simplified further by other transformations. [DAYA87, GANS87, MURA89] corrected errors in Kim s technique by replacing joins with outerjoins. DAYA87, MURA89] developed pipelining techniques that remove some of the temporary relations introduced by Kim s technique. MUMI90] uses this prior work to convert queries with nested query blocks into a series of queries ....

Richard A. Ganski and Harry K. T. Wong. Optimization of Nested SQL Queries Revisited. Proc. 1987 SIGMOD, May 1987.

....Another line of work has proceeded in an orthogonal direction, introducing execution plans which correspond to alternative structures of nesting. In particular, these approaches consider the possibilities of merging query blocks, denesting queries, and commuting aggregation blocks with joins [Day87, GW87, Kim82, Mur92, PHH92, YL94, HG94]. In this paper we propose an approach which unifies and generalizes the approaches mentioned above. We apply the selection propagation idea to certain data dependencies that are implicit in aggregation blocks. Propagation of SQL predicates [MFPR90a, MFPR90b, MPR90, LMS94] is a special case of ....

....dependencies that are implicit in aggregation blocks. Propagation of SQL predicates [MFPR90a, MFPR90b, MPR90, LMS94] is a special case of propagation of these dependencies. At the same time, propagating these 1 dependencies can produce execution plans with alternative nesting structure, as in [Day87, GW87, Kim82, Mur92, PHH92, YL94, HG94]. In addition to expressing in a common framework previously proposed query transformations which seemed unrelated, our approach incorporates naturally general data dependencies that may be given in the database schema. It extends transformations which commute joins with aggregation operators and ....

[Article contains additional citation context not shown here]

R. Ganski, H. Wong. Optimization of nested SQL queries revisited. In SIGMOD 1987.

....under limited circumstances. In spite of the acknowledged importance of such transformations, few systems have expanded upon these early transformation designs. Kim [Kim82] originally studied the question of when quantified subqueries could be replaced by joins (or anti joins) Ganski and Wong [GW87] and Dayal [Day87] did additional work on eliminating nested subqueries. These papers recognize the importance of merging of subqueries. Kim82, GW87] also deal with subqueries containing aggregation. We have reported our set of rules that deal with such subqueries in [MFPR90a, MPR90, MFPR90b] ....

....designs. Kim [Kim82] originally studied the question of when quantified subqueries could be replaced by joins (or anti joins) Ganski and Wong [GW87] and Dayal [Day87] did additional work on eliminating nested subqueries. These papers recognize the importance of merging of subqueries. Kim82, GW87] also deal with subqueries containing aggregation. We have reported our set of rules that deal with such subqueries in [MFPR90a, MPR90, MFPR90b] Ganski s paper illustrates the complexity of query rewrite, since it has to emend some previous transformations which were incorrect. This complexity ....

[Article contains additional citation context not shown here]

Richard A. Ganski and Harry K. T. Wong. Optimization of Nested SQL Queries Revisited. In Proc. ACMSIGMOD International Conference on Management of Data, pages 23--33, San Francisco, May 1987.

....one evaluation technique) except that tree scheme occurrences are used as a representation of (intermediate) results. Later, Day87] proposed the G Join, G Aggr and G Restr. The G Join replaces the graft operator and a sequence of G Aggr and G Restr replaces the previously used prune operator. [GL87] treated queries with quanti cation as a special case of nested queries. The quanti ers exists and not exists are replaced by count aggregations. More recently, Steenhagen et al. SABdB94] investigated rules for unnesting queries in an object oriented model. In this paper we begin with a ....

R. A. Ganski and H. K. T. Long. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 2233, San Francisco, USA, May 1987.

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R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.

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R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.

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R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.

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R. A. Ganski and H. K. T. Wong. Optimization of Nested SQL Queries Revisited. In ACM SIGMOD, 1987.

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R. A. Ganski and H. K. T. Wong. Optimization of nested SQL queries revisited. In SIGMOD, 1987.

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R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.

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R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.

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R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.

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R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.

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R. Ganski and H. Wong. Optimization of nested SQL queries revisited. In Proc. of the ACM SIGMOD Conf. on Management of Data, pages 23--33, 1987.

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8 Richard A. Ganski and Harry K. T. Wong. Optimization of Nested SQL Queries Revisited. In Proc. of the ACM SIGMOD Int'l Conference on Management of Data, pages 23--33, San Francisco, USA, 1987.

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