Usage

Physical Database DDL Statements › SEGMENT Statements › Entity Currency › Usage

Usage

Assigning Page Groups

When you assign a segment to a page group, keep these restrictions in mind:

For non-SQL defined databases, all data accessed within a run unit must be in the same page group and have the same maximum number of records per page unless you specify the MIXED PAGE GROUP BINDS ALLOWED option
When adding segments to a DMCL, areas within a page group must have unique, non-overlapping page ranges

CA IDMS/DB Rounds Up the Maximum Record Count

CA IDMS/DB may change the maximum number of records or rows that can be stored on a single page. CA IDMS/DB rounds the value to the next higher power of 2 less 1 to arrive at the actual number of records per page. (This is the largest number that can be represented in the same number of bits.) The following table shows the actual maximum records per page resulting from values specified for maximum-record-count.

Value specified in MAXIMUM RECORDS clause	Actual maximum records per page	High allowable page number
3	3	1,073,741,822
4 - 7	7	536,870,910
8 - 15	15	268,435,454
16 - 31	31	134,217,726
32 - 63	63	67,108,862
64 - 127	127	33,554,430
128 - 255	255	16,777,214
256 - 511	511	8,388,606
512 - 1,023	1,023	4,194,302
1,024 - 2,047	2,047	2,097,150
2,048 - 2,727	2,7271	1,048,574

Note: Although a 12-bit line number would theoretically accommodate 4,095 records per page, only 2,727 4-byte record occurrences can actually be stored on the largest possible page.

MAXIMUM RECORDS Clause Determines the Db-Key Format

Because the MAXIMUM RECORDS PER PAGE clause determines the number of bits required for a line number, it also determines the format of database keys for the segment. A database key is a 32-bit field, made up of 2 values:

The number of the page on which a record occurrence or row resides
The record's or row's line number within that page

Maximum-record-count determines the number of bits required to store a line number (minimum 2 bits; maximum 12); the remaining bits become the page-number portion of the database key. Consequently, maximum-record-count and the page numbers assigned to schema areas are dependent upon one another, as is shown in the table above.

In most cases, maximum-record-count can be left to default to 255; this accommodates a database with page numbers up to 16,777,214.

Note: The number specified in the MAXIMUM RECORDS clause indicates the maximum number of records that the run-time system will place on a single page. The actual number of records on a given page depends on the page size specified on the AREA statement and the sizes of individual records or rows placed on the page.

Note: For information about how the MAXIMUM RECORDS clause and the area's page size affect the number of records or rows on a page, see the presentation of space management in Space Management.