About access methods

An access method is a set of database server routines that HCL Informix® uses to access and manipulate a table or an index. The two types of access methods are primary and secondary.

Informix uses a primary access method to perform standard table operations, such as inserting, deleting, updating, and retrieving data.

Informix uses a secondary access method to build, use, and manipulate an index structure. Indexes are built on one or more columns of a table to provide a quick way to find rows in a database based on the value in the indexed column or columns.

The routines of a secondary access method encapsulate index operations, such as how to:

Build an index
Scan the index
Insert new information into an index as new data is inserted into the indexed table
Update an index as the indexed table is updated
Delete data from an index as data is deleted from the indexed table

These routines are collectively called purpose functions.

Secondary access methods are used in combination with operator classes, which describe when an access method can be used in a query and how to perform the index operations, such as scanning and updating. Operator classes are a way of specifying the routines that play particular roles in access-method operations. Operator classes are described in About operator classes.

Informix provides two secondary access methods:

B-tree, which stands for balanced tree. B-tree is the default secondary access method for ordered data values.
R-tree, which stands for range tree. R-tree is an access method for multidimensional (spatial) and interval data.

The B-tree access method is described in your HCL Informix Administrator's Guide.

Tip: Indexes that are created and manipulated by a particular secondary access method are referred to by the name of the access method. For example, the R-tree secondary access method is used to create and manipulate R-tree indexes.