A page group is an attribute of a segment. It uniquely identifies a collection of page ranges. For example, page 30,002 of page group 0 is different than page 30,002 of page group 1. The following diagram shows how page groups allow areas to be defined with the same or overlapping page ranges:
PAGE GROUP 0 PAGE GROUP 1 ┌───────────────────────┐ ┌───────────────────────┐ │ AREA EMP-AREA │ │ AREA ORDER-AREA │ │ PAGES 30000 to 30500│ │ PAGES 30000 to 30500│ └───────────┬───────────┘ └───────────┬───────────┘ │ │ │ │ ▼ ▼ PAGE 30,002 of PAGE 30,002 of PAGE GROUP 0 PAGE GROUP 1
The default page group, 0, allows you to use up to 16,777,214 database pages containing up to 255 records/rows per page. Typically, you use page groups if your database environment requires more than 16,777,214 database pages; for example, if you access multiple, large databases within a single DMCL. By using page groups, you can include areas with the same page range in a single DMCL.
You may define a database with a mix of page groups; however, you may not define a database in which a chain set crosses a page group boundary. For SQL-defined databases, neither indexes nor referential constraints may cross a page group boundary.
By default, a run unit can access data from only one page group at a time. This restriction can be overcome by specifying the MIXED PAGE GROUP option on the DBNAME statement that defines the database, but using this option has implications for programs accessing the database.
SQL sessions can access data in mixed page groups without any restrictions or special considerations.
Note: For more information about using mixed page groups, see the Chapter “Defining a Database Name Table”.
There are special rules about mixed page groups and dictionaries.
Note: For more information, see the Chapters “Defining a Database Name Table” and “Dictionaries and Runtime Environments”.
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