Database Concepts › Physical Structures › Location Mode › CALC Location Mode

CALC Location Mode

The CALC location mode requires the specification of a CALC key made up of one or more record elements. Usually, a CALC key is chosen so that its value is unique across all record occurrences, although this is not a requirement. At Commonweather, for example, EMPLOYEE records are stored CALC. The EMP-ID, a 4-digit number unique to each employee, is chosen as the CALC key element.

The CALC method is used for:

• Randomization- Records are distributed evenly over all the pages in the area using a randomizing algorithm, thereby minimizing overflow and leaving space for VIA occurrences.
• Direct Retrieval by Symbolic Key- A record occurrence can be retrieved with a single access using its CALC key (rather than by reading all records in an area or searching through an index).

A record whose location mode is CALC can be retrieved by simply specifying the value of its CALC key; DBMS automatically converts the CALC key into the same target page member as that used to store the record. If the CALC record had overflowed to another page when it was added to the database, the DBMS can locate it using an internally maintained set called a CALC set whose owner resides on the record's original target page.