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Extending Informix®Beyond standard relational database objects, HCL Informix® can be extended to handle specialized data types, access methods, routines, and other objects. Informix® includes many built-in extensions that are fully integrated in the database server. Informix® also provides DataBlade® modules, which are packages of extended database objects for a particular purpose and that are installed separately from the database server. Alternatively, you can create your own user-defined objects for Informix®.
Creating extensionsYou can create user-defined data types, routines, access methods, and other database objects to suit your needs. You can use application programming interfaces to write user-defined routines and applications that access data in Informix® databases.
Virtual-Index Interface GuideThe Informix® Virtual-Index Interface Programmer's Guide explains how to create a secondary access method with the Virtual-Index Interface (VII) to extend the built-in indexing schemes of HCL Informix® typically with a DataBlade® module.
Design decisionsThese topics present the choices that you make to optimize the performance and flexibility of the access method that you can create with the virtual-index interface (VII).
Process queries that involve a virtual indexThis section describes various options for processing a SELECT statement, or query, that involves a virtual index.
Qualify dataTo qualify table rows, a secondary access method applies the functions and Boolean operators from the qualification descriptor to key columns. The access method actually retrieves the contents of the keys from an index rather than from the table. If the index keys qualify, the secondary access method returns identifiers that enable the database server to locate the whole row that includes those key values.
Support for query plan evaluationAt the start of a SELECT statement, the database server initiates query planning. A query plan specifies the steps that the database server takes to fulfill a query with optimal efficiency.
Update statisticsThe UPDATE STATISTICS statement stores statistics about the distribution of rows on physical storage media for use by the optimizer. The database server updates data-distribution statistics for internal, relational tables; the access method updates data-distribution statistics for virtual indexes.

Update statistics

The UPDATE STATISTICS statement stores statistics about the distribution of rows on physical storage media for use by the optimizer. The database server updates data-distribution statistics for internal, relational tables; the access method updates data-distribution statistics for virtual indexes.

When a user issues an UPDATE STATISTICS statement that requires the access method to determine the distribution of data in a table, the database server calls the am_stats purpose function.

The access method can call mi_tab_update_stat_mode() to determine if the UPDATE STATISTICS statement includes the keyword HIGH or MEDIUM, each of which influences the percentage of rows that the access method should sample and the particular statistics that it should supply.

To store statistics in the statistics descriptor, the am_stats purpose function calls the various accessor functions with the name prefix mi_tstats_set. The database server copies the information from the statistics descriptor in the appropriate system catalog tables.

For information about the effects of query costs and distribution of data, see the Informix® Performance Guide.

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