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

ID: Q136765

The information in this article applies to:

Microsoft Visual FoxPro for Windows, version 3.0

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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