Descriptor reference
Descriptors are the predefined data structures through which the database server and access method pass information.
The application programming interface (API) that the HCL Informix® database
server provides with the Virtual-Table Interface (VTI) consists primarily
of the following components:
- Opaque data structures, called descriptors, that the database server passes by reference to purpose functions
- Accessor functions that store and retrieve descriptor values
The Virtual-Table Interface (VTI) provides the following
descriptors and accessor functions.
| Descriptor | Describes | Accessor-function prefix |
|---|---|---|
| qualification descriptor (MI_AM_QUAL_DESC) | WHERE clause criteria | mi_qual_ |
| parameter descriptor (MI_AM_PARAM_DESC) | Parameters in the qualifier of the WHERE clause | mi_qual_param_ |
| row descriptor (MI_ROW) | Order and data types of projected columns | Various DataBlade® API functions |
| scan descriptor (MI_AM_SCAN_DESC) | Projection clause lists objects or expressions to retrieve | mi_scan_ |
| statistics descriptor (MI_AM_TSTATS_DESC) | Distribution of values | mi_tstats_ |
| table descriptor (MI_AM_TABLE_DESC) | Table attributes and fragment partition | mi_tab_ |
Important: Because the internal structure
of any VTI descriptor might change, HCL Informix declares
them as opaque structures. To make a portable access method, always
use the access functions to extract or set descriptor values. Do not
access descriptor fields directly.