Category: ASE: Data Storage

large object page, LOB page

Large object columns (LOB columns), i.e. text, image and Java off-row columns of a table are stored in a separate data structure, a so-called LOB structure. Every table, independent of how many text, image and off-row columns it contains, is set up in this data structure.

The table itself has a 16 bytes pointer to the page storing the first LOB value. The first page contains information on the number of bytes in the LOB value, i.e. the size of the LOB value. A LOB value which is distributed over several pages is called a page chain. The last page used for the LOB value in a page chain contains the so-called NULL-next-page pointer. The NULL-next-page pointer marks the end of the preceeding and the beginning of the next page chain.

This LOB data structure is listed in the system table sysindexes. The column id contains the identification number of the LOB structure. It is identical to the table id. FOr LOB structures, the column indid always shows a value of "255". The column name contains the name of the table in which the LOB value is stored, prefixed by the letter "t".

Every LOB page can store up to 1800 bytes. Every non-zero value reserves at least one page.

Reading and Writing LOB values