SET DCACHE

Purpose

Map table into memory

Syntax

SET DCACHE TO [<expN>] [OPTIMUM]

SET DCACHE ON | OFF | (<expL>)

See Also

SET CACHELOAD, SET ICACHE, SET SQLCACHE

Description

If SET DCACHE is ON when a table is used, then the table records will be ’cached’ in memory, and the records which are not held in memory will be read in when they are needed.

The SET DCACHE TO <expN> command allows the number of records held in memory to be specified on a per table basis. If the optional OPTIMUM keyword is specified, the optimum dcache size will automatically be set. The table ’cache ’ area acts as a ’window’ into the table. Each record referenced in the table is ’mapped’ into the table cache using the simple formula:

position = mod(<record number>, <cache size>)

Using this formula with a table cache of 500, record 1 maps into slot 1, record 501 also maps into slot 1. When a slot is referenced which already contains a record, and that record has been modified since it was read from the table, then the table is updated with the modified record and the required record is read into the cache slot. When a table is closed, modified records held in memory are written into the table.

The SET DCACHE command has no effect if a table is shareable. The SET CACHELOAD command can be used in association with the SET DCACHE command. This forcibly loads the table and index cache when the table is used exclusively. The SET ICACHE command can be used to ’cache’ index keys in memory. By default, DCACHE is ON for the SQL Engine (for temporary resultsets, which are not shared) and OFF otherwise.

Example

// Open up payroll system
set exclusive on
set cacheload on
set icache to 3000
set dcache to 1000
use payroll index pay_date, emp_code
// Cache the employee records in memory
set dcache on
use employees index name, emp_code
set dcache off
use wages index emp_code