Correlated Tables for Nested Query Evaluation
نویسنده
چکیده
In this paper, we first define the correlated table of an SQL query without nested subqueries which extends the result set of an SQL query to a set of result sets, one for each distinct binding tuple. We then present an algorithm to convert any SQL query with nested subqueries to a correlated query without nested subqueries. The proposed algorithm provides a simple but very effective method unnest nested queries and thus can be used to efficiently evaluate nested queries in distributed database systems. Keyword: query evaluation, distributed databases, nested queries ————————————————
منابع مشابه
Using Nested Tables for Representing and Querying Semistructured Web Data
This paper proposes an approach for representing and querying semistructured Web data, which is based on nested tables allowing internal nested structural variations. Our motivation is to reduce the complexity found in typical query languages for semistructured data and to provide users with an alternative for quickly querying data obtained from multiple-record Web pages. We show the feasibilit...
متن کاملUnnesting Arbitrary Queries
SQL-99 allows for nested subqueries at nearly all places within a query. From a user’s point of view, nested queries can greatly simplify the formulation of complex queries. However, nested queries that are correlated with the outer queries frequently lead to dependent joins with nested loops evaluations and thus poor performance. Existing systems therefore use a number of heuristics to unnest ...
متن کاملOptimization of Nested Queries in a Distributed Relational Database
This paper describes how nested queries in the SQL language are processed by R*, an experimental adaptation to the distributed environment of the well-known centralized relational DBMS, System R. Nested queries are queries in which a predicate references the result of another query block (SELECT...FROM...WHERE...), called a subquery block (subQB). SubQBs may themselves contain one or more subQB...
متن کاملExecuting Nested Queries
Optimization of nested queries, in particular finding equivalent “flattened” queries for queries that employ the SQL sub-query construct, has been researched extensively. In contrast, with the exception of nested loops join, execution of nested plans has found little interest. Nested execution plans may result from a failure to flatten nested SQL expressions but just as likely are created by a ...
متن کاملFrom Nested-Loop to Join Queries in OODB
Most declarative SQL-like query languages for object-oriented database systems (OOSQL) are orthogonal languages allowing for arbitrary nesting of expressions in the select-, from-, and where-clause. Expressions in the from-clause may be base tables as well as setvalued attributes. In this paper, we propose a general strategy for the optimization of nested OOSQL queries. As in the relational mod...
متن کامل