Column and Parameter Descriptors

This section presents application and implementation descriptors as implemented by DB Link. It is divided as follows:

Notion of Descriptors

CLI Definition

The CLI standard defines descriptors at implementation- and application- levels.

  • At implementation level —that is, from the database server point of view— the descriptors are called IRD (Implementation Row Descriptor) and IPD (Implementation Parameter Descriptor).

  • At application level, the descriptors are called ARD (Application Row Descriptor) and APD (Application Parameter Descriptor).

Likewise, DB Link also differentiates between implementation level and application level, but refers to row descriptors as column descriptors and uses the same classes for column and parameter descriptors.

Implementation Within DB Link

DB Link uses two classes to describe the properties of a column or parameter:

  • IldDescriptor: Declared in the file ildent.h, this class is used to describe a column or parameter type on the server side.

  • IldAppDescriptor: A subclass of IldDescriptor declared in the file ildtuple.h, this class is used to hold the type descriptor and the column or parameter values and indicators on the application side.

Implementation Descriptors

An instance of IldDescriptor holds the following information about the column or parameter. IldDescriptor Member Functions shows the corresponding member functions:

IldDescriptor Member Functions

Use this Member Function

To get

IldDescriptor::getType

DB Link type

IldDescriptor::getSqlType

SQL data type code

IldDescriptor::getName

Column name

IldDescriptor::getSize

Maximum data size (in bytes)

IldDescriptor::getPrecision

Precision (for columns of a numeric type) - 0 if irrelevant

IldDescriptor::getScale

Scale (for columns of a numeric type) - 0 if irrelevant

IldDescriptor::getSqlTypeName

SQL type name in the server

IldDescriptor::isNullable

Nullability flag

IldDescriptor::getADTDescriptor

Abstract type descriptor (for columns of DB Link type) - null otherwise
Note: Since most RDBMSs do not have the capability of describing the parameters of a query, the contents of the IldDescriptor object for a parameter are undefined until it is bound using the member function bindParam.

Type Codes

SQL data type codes are defined as constants in the section Type Codes of the ildconst.h header file. Most of them have a name that is very similar to their CLI name but a few of them differ. The names are prefixed with IldSQL.

Their values are the ones defined in the CLI standard.

The CLI specification has been extended to negative values so as to provide support for all types of all supported RDBMSs.

Application Descriptors

An instance of IldAppDescriptor adds the following information to its base class IldDescriptor:

Member functions added by IldAppDescriptor to IldDescriptor

Information

Member Function

Size of one element in the value buffer (in bytes)

getBufferSize

Value buffer

getValue

Indicator buffer

getNulls

In addition:

  • The member function isExtValue lets you know whether the value buffer was allocated by DB Link or is bound to some application memory space. This function returns IlFalse in the first case or IlTrue in the second.

  • The member function isExtNulls lets you know whether the null-indicator buffer was allocated by DB Link or is bound to some application memory space. This function returns IlFalse in the first case or IlTrue in the second.

Simple IldDescriptor objects exist only for user-defined data-type attribute descriptors of type IldADTDescriptor. All other accesses to descriptors return instances of derived classes: