Nonstop Database, HiRDB Version 9 Description
This section describes page statuses and explains page design policy.
The following table lists and describes the statuses that are assigned to pages.
Table 4-3 Page statuses
Page status | Description |
---|---|
Unused page | A page that has not been assigned. |
Used free page | A page in which no data is being stored, as a result of data deletion.# |
Active page | A page on which data is being stored, and that has free space to which data can be added. For a table on which the free space reusage facility is being used, its active pages include those pages to which data could not be added because the free space on the page could not be used due to data deletion.# |
Used full page | A page on which data is being stored, but that has no free area to which data can be added. For a table or index on which the free space reusage facility is not being used, its used full pages include those pages to which data could not be added because the free space on the page could not be used due to data deletion.# |
Pages must be designed with care, because the page size affects data I/O times. For details about page design, see the HiRDB Version 9 Installation and Design Guide.
When you define a table or index, you can specify the ratio of unused area in a page. This concept is illustrated in the following figure.
Figure 4-4 Ratio of unused area in a page
The data storage efficiency improves as the page unused area ratio becomes smaller. Increasing the page unused area ratio sometimes improves performance. For example, when data is updated, you can reduce the number of data I/O operations, provided that either of the following occurs:
For details about determining the page unused area ratio, see the HiRDB Version 9 Installation and Design Guide.
Pages are not allocated when the table is defined. Instead, pages are allocated as needed when data is saved to the table. Once a page is allocated (once it becomes a used page), it cannot be reused until it is released.
When an index is defined, the system allocates pages according to the number of data items to be included in the index. If there are no data items, only one page (root page) is allocated. However, if the EMPTY option is specified in the CREATE INDEX (index entries not to be created), no pages will be allocated.
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