Dynamic select/omit

 

With the dynamic select/omit operation, the system only returns those records that meet the select/omit values when a program reads records from the file. That is, the actual select/omit processing is done when records are read by a program, rather than when the records are added or changed.

However, the keyed sequence access path contains all the keys, not just keys from selected records. Access paths using dynamic select/omit allow more access path sharing, which can improve performance.

To specify dynamic select/omit, use the Dynamic selection (DYNSLT) keyword. With dynamic select/omit, key fields are not required.

If you have a file that is updated frequently and read infrequently, you might not need to update the access path for select/omit purposes until your program reads the file. In this case, dynamic select/omit might be the correct choice. The following example helps describe this.

You use a code field (A=active, I=inactive), which is changed infrequently, to select/omit records. Your program processes the active records and the majority (over 80%) of the records are active. It can be more efficient to use DYNSLT to dynamically select records at processing time rather than perform access path maintenance when the code field is changed.

 

Parent topic:

Selecting and omitting records for logical files

 

Related concepts

Sharing existing access paths between logical files