Specifying the current access path maintenance (MAINT) parameter when creating database files

The MAINT parameter specifies how access paths are maintained for closed files. While a file is open, the system maintains the access paths as changes are made to the data in the file. However, because more than one access path can exist for the same data, changing data in one file might cause changes to be made in access paths for other files that are not currently open (in use). The three ways of maintaining access paths of closed files are:

• Immediate maintenance of an access path means that the access path is maintained as changes are made to its associated data, regardless if the file is open. Access paths used by referential constraints will always be in immediate maintenance.
• Rebuild maintenance of an access path means that the access path is only maintained while the file is open, not when the file is closed; the access path is rebuilt when the file is opened the next time. When a file with rebuild maintenance is closed, the system stops maintaining the access path. When the file is opened again, the access path is totally rebuilt. If one or more programs has opened a specific file member with rebuild maintenance specified, the system maintains the access path for that member until the last user closes the file member.
• Delayed maintenance of an access path means that any maintenance for the access path is done after the file member is opened the next time and while it remains open. However, the access path is not rebuilt as it is with rebuild maintenance. Updates to the access path are collected from the time the member is closed until it is opened again. When it is opened, only the collected changes are merged into the access path.

If you do not specify the type of maintenance for a file, the default is immediate maintenance.

MAINT Parameter Comparison

Table 1 compares immediate, rebuild, and delayed maintenance as they affect opening and processing files.

Table 1. MAINT Values

Function	Immediate Maintenance	Rebuild Maintenance	Delayed Maintenance
Open	Fast open because the access path is current.	Slow open because access path must be rebuilt.	Moderately fast open because the access path does not have to be rebuilt, but it must still be changed. Slow open if extensive changes are needed.
Process	Slower update/output operations when many access paths with immediate maintenance are built over changing data (the system must maintain the access paths).	Faster update/output operations when many access paths with rebuild maintenance are built over changing data and are not open (the system does not have to maintain the access paths).	Moderately fast update/output operations when many access paths with delayed maintenance are built over changing data and are not open, (the system records the changes, but the access path itself is not maintained).
Note: Delayed or rebuild maintenance cannot be specified for a file that has unique keys. Rebuild maintenance cannot be specified for a file if its access path is being journaled.

MAINT Parameter Tips

The type of access path maintenance to specify depends on the number of records and the frequency of additions, deletions, and updates to a file while the file is closed.

You should use delayed maintenance for files that have relatively few changes to the access path while the file members are closed. Delayed maintenance reduces system overhead by reducing the number of access paths that are maintained immediately. It may also result in faster open processing, because the access paths do not have to be rebuilt.

You may want to specify immediate maintenance for access paths that are used frequently, or when you cannot wait for an access path to be rebuilt when the file is opened. You may want to specify delayed maintenance for access paths that are not used frequently, if infrequent changes are made to the record keys that make up the access path.

In general, for files used interactively, immediate maintenance results in good response time. For files used in batch jobs, either immediate, delayed, or rebuild maintenance is adequate, depending on the size of the members and the frequency of changes.