Record Storage and Deletion › Record Deletion › Logical Deletion

Logical Deletion

Pointers Deleted

To avoid consuming unnecessary time and I/O disconnecting records from sets without prior pointers, CA IDMS/DB does not physically delete the record when an ERASE command is issued. Instead, the next time CA IDMS/DB encounters a logically deleted record while walking a chained set of which the record is a member, CA IDMS/DB disconnects the record from the set, provided that the record's area was readied in update mode. Since the record prior to the logically deleted record is still current of run unit, CA IDMS/DB can update the record's next pointer and disconnect the logically deleted record. To be physically deleted, the record must have been disconnected from all sets in which the record was a member.

Operations Performed

CA IDMS/DB performs the following operations to logically delete a record:

• Removes the record's data from the database, but leaves the prefix
• Moves all records following the deleted record on the page, so that all free space remains in the middle of the page
• Sets the logical delete flag (the first bit) in the record id field of the record's line index
• Updates the space available count field in the header

Example

In the following example, assume that the EMPOSITION records do not have prior pointers in the EMP-EMPOSITION set. When erasing an EMPOSITION record, CA IDMS/DB removes only the data and flags the record's line index. The EMPOSITION record is logically deleted. The next time CA IDMS/DB is walking this occurrence of the EMP-EMPOSITION set in update mode and encounters the flagged record, CA IDMS/DB physically deletes the record.

Consideration

Occasionally, in recovering from an error during a store operation, CA IDMS/DB may create a logically deleted record. If CA IDMS/DB has stored a record and is in the process of making the automatic connections when CA IDMS/DB discovers an error condition (for example, no currency established in one of the automatic sets), CA IDMS/DB must erase the record being stored. If one of the chained sets to which the record has already been connected has next pointers only, CA IDMS/DB logically deletes the record.