User Guide

SAP AG November 2002

Secondary Key

Secondary keys can be defined for each table to optimize the data access via SQL

statements. They can refer to any column combination and they help to prevent sequential

scans over the table. Like the primary key [Page 101], the secondary key can consist of

multiple columns.

A secondary key is often called an index (not to be confused with B* index).

B* Tree

All SAP DB data is stored in structures called B* trees. The B* tree method is far more

efficient for accessing the rows of a table than other access methods (for example, sequential

scans).

A B* tree spans several levels from the root level at the top, to the index levels, through to

the lowest leaf level (one root level, n index levels (n>=1), one leaf level).

The storage units are called pages or data pages. SAP DB Version 7 supports a page size of

8 KB.

In SAP DB Version 7, all subsequent comparisons in a B* tree use ASCII code (each

character is assigned a numerical value between 0 and 255). The comparisons are performed

for all values on a character-by-character basis.

The sort criterion for building a B* tree is the primary key [Page

101].

B* Tree

Au Az Baf ... ... Bi

Waf

...

Aalen Amiens Anizy Athens Auber Avon

.. .. ..

Root level

Index level

Leaf level