User-defined secondary-access methods

If the concepts of less than, greater than, and equal do not apply to the data to be indexed, you might consider using a user-defined secondary-access method instead of the built-in secondary-access method, which is a B-tree index. You can use a user-defined secondary-access method to access other indexing structures, such as an R-tree index.

If your database supports a user-defined secondary-access method, you can specify that the database server uses this access method when it accesses a particular index. For information about how to determine the secondary-access methods that your database defines, see Identifying the available access methods.

To choose a user-defined secondary-access method, use the USING clause of the CREATE INDEX statement. The USING clause specifies the name of the secondary-access method to use for the index you create. This name must be listed in the am_name column of the sysams system catalog table and must be a secondary-access method (the am_type column of sysams is 'S').

The secondary-access method that you specify in the USING clause of CREATE INDEX must already be defined in the sysams system catalog. If the secondary-access method has not yet been defined, the CREATE INDEX statement fails.

When you omit the USING clause from the CREATE INDEX statement, the database server uses B-tree indexes as the secondary-access method. For more information, see the CREATE INDEX statement in the HCL Informix Guide to SQL: Syntax.


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