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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.
Descriptor function reference
Descriptors
Statistics descriptorAn access method returns statistics to the UPDATE STATISTICS statement in a statistics descriptor, or MI_AM_ISTATS_DESC structure. The database server copies the separate values from the statistics descriptor to pertinent tables in the system catalog.

Statistics descriptor

An access method returns statistics to the UPDATE STATISTICS statement in a statistics descriptor, or MI_AM_ISTATS_DESC structure. The database server copies the separate values from the statistics descriptor to pertinent tables in the system catalog.

The following accessor functions set information in the statistics descriptor.


Accessor function	Value set
mi_istats_set2lval()	A pointer to the second largest key value in the index
mi_istats_set2sval()	A pointer to the second smallest key value in the index
mi_istats_setclust()	The degree of clustering A low number indicates fewer clusters and a high degree of clustering.
mi_istats_setnleaves()	The number of leaves in the index
mi_istats_setnlevels()	The number of levels in the index
mi_istats_setnunique()	The number of unique keys in the index

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