Navigator User Guide

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Using VSAM Keys

PowerExchange reads key information for VSAM data sets from the catalog when the data set is allocated and opened.

Both the key length and the relative key position are retrieved.

For example, the following lines define a record layout for keypos=10 and keylen=4:

01 REC.
    04 HEADER     PIC X(10).
    04 KEY.
        08  KEY1     PIC 99. 
        08  KEY2     PIC 99.
    04 REST          PIC X(200).

For optimization to work, include KEY1 in the WHERE clause.

Use key information to speed processing of WHERE clauses, which provides optimization.

Different WHERE clause formats produce different results.

For example:

Line 1    SELECT * FROM TEST.VSAM1.TAB WHERE (KEY1 > 50) AND (KEY1 < 90)

Line 1 translates to:

(50 < KEY1 < HIGH-HALUES) AND (LOW-VALUES < KEY1 < 90)

The following example generates the same results as the Line 1 example, but with a different elapsed time:

Line 2    SELECT * FROM TEST.VSAM1.TAB WHERE (KEY1 > 50  AND  KEY1 < 90)

Line 2 translates to:

(KEY1 > LOW-VALUES AND KEY1 < HIGH-HALUES)

This format is still technically optimized, but when the file is processed, the read start is positioned at LOW-VALUES and read until HIGH-VALUES, which is not ideal.

Line 2 is the preferred syntax. This syntax positions the file at record 50 and reads data until KEY1 is greater than or equal to 90.

Optimization is used for key positioning, the data is selected against the WHERE clause to verify that the correct records are filtered.

If you specify a value greater than 0 in the

Skip First Records from File

box in the

KSDS Access Method

dialog box, the SELECT statement is not optimized even if you specify KEY1.

Improving Bulk Read Performance for VSAM Data Sets

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Table of Contents

Navigator User Guide

Navigator User Guide

Using VSAM Keys

Using VSAM Keys