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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.
DataBlade® API Programmer's GuideThe Informix® DataBlade® API Programmer's Guide describes information about the DataBlade® API, the C-language application programming interface (API) provided with HCL Informix®.
Create user-defined routines
Create special-purpose UDRsThis section describes how to create special UDRs, such as iterator functions, user-defined aggregates, and optimization functions.
Provide UDR-optimization functions
Write selectivity and cost functionsThe query optimizer uses the selectivity and cost of a query to help select the best query plan. To help the optimizer select the best query plan, you can provide the query optimizer with information about the selectivity and cost of your UDR.
Query selectivityIf your UDR is a Boolean function, it can be used as a filter in a query. The query optimizer uses the selectivity of a query to estimate the number of rows that the query will return. The selectivity is a floating-point value between zero and one that represents the percentage of rows for which the query (and each filter in the query) is expected to return a true value.

Query selectivity

If your UDR is a Boolean function, it can be used as a filter in a query. The query optimizer uses the selectivity of a query to estimate the number of rows that the query will return. The selectivity is a floating-point value between zero and one that represents the percentage of rows for which the query (and each filter in the query) is expected to return a true value.

For a list of Boolean functions that are useful as query filters, see Boolean functions useful as query filters.

For a Boolean function likely to be used as a query filter, you can use the following routine modifiers to specify a selectivity for the function.


Selectivity	Routine modifier
Selectivity is constant for every invocation of the Boolean function.	SELCONST = selectivity_value selectivity_value is a floating-point value between 0 and 1.
Selectivity varies according to some execution conditions.	SELFUNC = selectivity_func selectivity_func is the name of a selectivity function that returns a floating-point value between 0 and 1 to indicate the selectivity of the Boolean function.

When the query optimizer needs to determine the selectivity of the Boolean function, it either uses the constant selectivity value or calls the selectivity function, depending whether the Boolean function was registered with the SELCONST or SELFUNC routine modifier.

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