Index overview

An index is created on one or more table columns and points to the place where the column data is stored on disk. Indexes speed up data retrieval and are useful for enforcing referential integrity. A table can have more than one index.

The index attributes you can define when you create an index with Sybase Central are:

Clustered or nonclustered
Unique or nonunique

A clustered index is sorted on an ongoing basis so that the physical order of rows is the same as the logical order. The bottom or leaf level of the index contains the actual data pages of the table. Finding data using a clustered index is faster than using a nonclustered index.

Create a clustered index before creating nonclustered indexes because nonclustered indexes are rebuilt when a clustered index is created. A table can have only one clustered index.

A nonclustered index stores key values and pointers to data pages rather than the data itself. The physical order of the rows is not the same as the indexed order. A table can have up to 249 nonclustered indexes.

A unique index is one in which no two rows can have the same index value. A unique index is useful as an integrity check on unique data.

A nonunique index is one in which two or more rows can have the same index value.

You can combine these characteristics; for example, you can create a unique, nonclustered index.

After you create an index, you can change the segment on which it is allocated and you can check index consistency. When you modify the index, the old index is deleted from the database and replaced with the new version.

A function-based index contains one or more expressions as index keys. Like computed column indexes, this feature is useful for user-defined ordering and DDS applications.