Retrievals Supported by ISAM
ISAM can limit a scan if you specify at least the leftmost part of the key for the desired rows. ISAM also limits the pages scanned if you are looking for ranges of the key.
• If the key is a character key, ISAM supports character matching with limited scan if you specify at least the leftmost part of the key.
• If the key is a multi-column key, ISAM limits the pages scanned only if you specify at least the leftmost part of the key.
For instance, assume you modified the employee table to ISAM on name and age using the Structure of Table dialog. Alternatively, you can use the following MODIFY statement:
modify employee to ISAM on name, age;
The following retrievals make use of the ISAM key:
select * from employee
where employee.name like 'S%';
select * from employee
where employee.name = 'Shigio'
and employee.age > 30;
In contrast, the following retrievals do not make use of the ISAM key, because the leftmost part of the key (name) is not restricted:
select * from employee
where employee.age = 32;
select * from employee
where employee.name like '%S'
and employee.age = 32;
select * from employee
where employee.name like '%higio%';
Last modified date: 08/28/2024