Category: ASE: Data Storage

data pages, index pages

data pages and index pages are logical pages to store indexes or other objects like tables.

page header

Every data or index page contains a header, which stores information like e.g. "previous page pointer" for the object, "next page pointer" for the object, "page id number", the "object id" to which the page belongs and data for storage administration of the page. For objects with APL locking scheme the header is 32 bytes large. With the DOL locking scheme the header is uses 46 bytes.

offset table

On DOL tables data pages and index pages contain a so called offset table. The offset table stores pointers, referencing the starting byte of each data row on a page. Every pointer is two bytes large. The very first pointer is located on the end of a page. It occupies the last bytes of a page. Following pointers fill the page from back to front. Index pages on APL tables have no offset table.

data rows

data rows, are created directly after the 46 bytes large header of a page and fill the page from front to back. Only with the datatypes text and image as well as with Java off-row columns, data rows can be spread over several pages.

A data row leaves an overhead of at least four bytes. Rows storing variable length data can have a larger overhead.