About Access Methods

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

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

Dynamic Server 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 that 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 more detail in the section About Operator Classes.

Dynamic Server 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 IBM Informix: Administrator's Guide.