The data pages in an APL table are linked into a list of pages by pointers on each page. Pages in data-only-locked tables are not linked into a page chain.
In an allpages-locked table, each page stores a pointer to the next page in the chain and to the previous page in the chain. When you insert new pages, the pointers on the two adjacent pages change to point to the new page. When Adaptive Server scans an allpages-locked table, it reads the pages in order, following these page pointers.
Pages are also doubly linked at each index level of allpages-locked tables, and at the leaf level of indexes on data-only-locked tables. If an allpages-locked table is partitioned, there is one page chain for each partition.
Unlike allpages-locked tables, data-only-locked tables typically do not maintain a page chain, except immediately after you create a clustered index. However, this page chain is broken the first time you issue a command on the table.
When Adaptive Server scans a data-only-locked table, it uses the information stored in the OAM pages. See “Object allocation map pages”.
Another difference between allpages-locked tables and data-only-locked tables is that data-only-locked tables use fixed row IDs. This means that row IDs (a combination of the page number and the row number on the page) do not change in a data-only-locked table during normal query processing.
Row IDs change only when one of the operations that require data-row copying is performed, for example, during reorg rebuild or while creating a clustered index.
For information on how fixed row IDs affect heap operations, see “Deleting from a data-only locked heap table” and “Data-only-locked heap tables”.
