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Q136765: How Memory & Disk Space Are Used During an SQL SELECT Command

Article: Q136765
Product(s): Microsoft FoxPro
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 25-AUG-1999

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 3.0 
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SUMMARY
=======

This article explains how Visual FoxPro uses memory and disk space during an SQL
SELECT command.

MORE INFORMATION
================

SQL uses as much internal memory as it can for file caching (read buffering),
sorting (for ORDER BY, GROUP BY, SELECT DISTINCT, and UNION DISTINCT), and
performing joins. This memory is limited by the setting of SYS(3050).

When executing complex queries, SQL sometimes generates intermediate temporary
tables. These temporary tables consist of the result of one of the following:

- A JOIN of two source tables (or previous intermediate results).

  -or-

- A SORT of a source table (or previous intermediate result) to satisfy an
  ORDER BY, GROUP BY, SELECT DISTINCT, or UNION DISTINCT.

In many cases, a join and a sort or multiple sorts can be combined into one pass,
removing an intermediate temporary table result.

In addition, temporary files are used while performing a sort if the entire sort
cannot be performed in memory. The size of the temporary sort files for a given
sort should be no larger than:

  (record length + key length) * (number of records) * 2

In addition, SQL creates a temporary index file to perform a join if no index
exists. Creating the index may create temporary files also.

Additional query words: VFoxWin

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Keywords          :  
Technology        : kbVFPsearch kbAudDeveloper kbVFP300

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