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Overview
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Access Path Selection in a Relational Database Management
System
Selinger P. Griffiths M. M. Astrahan D. D. Chamberlin ‘,.::' It. A. Lorie T. G. Price 4: IBM Research
Division,
San Jose,
.
95193
retrieval. Nor does a user specify in what order joins are to be performed. The System R optimizer .chooses both join order and an access path for each table in the SQL statement. Of the many possible choices, the optimizer the chooses one which minimizes "total access cost" for performing the entire statement.
ABSTRACT: In a high level query and data manipulation language such as SQL, requests stated non-procedurally, without are reference to access paths. This paper describes how System R chooses access paths both simple (single relation) and for (such as joins), complex queries given a specification of desired data as a user of predicates. System R boolean expression database management is an experimental system developed to carry out research on the relational model of data. System R was designed and built by members of the IBM San Jose Research'Laboratory. 1.
California
This paper issues of will address the access path selection for queries. Retrieval for data manipulation (UPDATE, DELETE) is treated similarly. Section 2 will describe the place of the optimizer in processing the of a SQL statement, and section 3 will describe the storage component access paths that are available on a single physically stored table. In section optimizer cost intro4 the formulas are duced for single table queries, and section more 5 discusses the joining of two or and their corresponding costs. tables, Nested queries (queries in predicates) are covered in section 6.
Introduction
System' R is an experimental database based on the relational management system model of data which has been under development at the IBM San Jose Research Laborato1975 Cl>. The software was since ry developed as a research vehicle in relaand is tional database, not generally outside the IBM Research available Division.
2.
processi.Bg
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B.B u
statement
four A SQL statement is subjected to phases of Depending on 'the processing. origin and contents of the statement., these phases may be separated by arbitrary time. In System intervals. of RI these arbitrary time intervals are transparent to a SQL components which process the system These mechanisms and a descripstatement. the processing tion of of SQL statements terminals are both programs and from Only an overview further discussed in <2>. of those processing steps that are relevant to access path selection will be discussed here.