5.10. FILTER

A BLOB FILTER filter is a database object that is actually a special type of external function, with the sole purpose of taking a BLOB object in one format and converting it to a BLOB object in another format. The formats of the BLOB objects are specifed with user-defined BLOB subtypes.

External functions for converting BLOB types are stored in dynamic libraries and loaded when necessary.

For more details on BLOB subtypes, see Binary Data Types.

5.10.1. DECLARE FILTER

Used for

Declaring a BLOB filter to the database

Available in

DSQL, ESQL

Syntax

DECLARE FILTER filtername
  INPUT_TYPE <sub_type> OUTPUT_TYPE <sub_type>
  ENTRY_POINT 'function_name' MODULE_NAME 'library_name'
<sub_type> ::= number | <mnemonic>
<mnemonic> ::=
    BINARY | TEXT | BLR | ACL | RANGES
  | SUMMARY | FORMAT | TRANSACTION_DESCRIPTION
  | EXTERNAL_FILE_DESCRIPTION | user_defined

The DECLARE FILTER statement makes a BLOB filter available to the database. The name of the BLOB filter must be unique among the names of BLOB filters.

Specifying the Subtypes

The subtypes can be specified as the subtype number or as the subtype mnemonic name. Custom subtypes must be represented by negative numbers (from -1 to -32,768). An attempt to declare more than one BLOB filter with the same combination of the input and output types will fail with an error.

INPUT_TYPE

clause defining the BLOB subtype of the object to be converted

OUTPUT_TYPE

clause defining the BLOB subtype of the object to be created.

INSERT INTO RDB$TYPES (RDB$FIELD_NAME, RDB$TYPE, RDB$TYPE_NAME)
VALUES (‘RDB$FIELD_SUB_TYPE’, -33, ‘MIDI’);

Parameters

ENTRY_POINT

clause defining the name of the entry point (the name of the imported function) in the module.

MODULE_NAME

The clause defining the name of the module where the exported function is located. By default, modules must be located in the UDF folder of the root directory on the server. The UDFAccess parameter in firebird.conf allows editing of access restrictions to filter libraries.

Any user connected to the database can declare a BLOB filter.

Examples of FILTER

1. Creating a BLOB filter using subtype numbers.

   DECLARE FILTER DESC_FILTER
     INPUT_TYPE 1
     OUTPUT_TYPE -4
     ENTRY_POINT 'desc_filter'
     MODULE_NAME 'FILTERLIB';

2. Creating a BLOB filter using subtype mnemonic names.

   DECLARE FILTER FUNNEL
     INPUT_TYPE blr OUTPUT_TYPE text
     ENTRY_POINT 'blr2asc' MODULE_NAME 'myfilterlib';

See also

DROP FILTER

5.10.2. DROP FILTER

Used for

Removing a BLOB filter declaration from the database

Available in

DSQL, ESQL

Syntax

DROP FILTER filtername

The DROP FILTER statement removes the declaration of a BLOB filter from the database. Removing a BLOB filter from a database makes it unavailable for use from that database. The dynamic library where the conversion function is located remains intact and the removal from one database does not affect other databases in which the same BLOB filter is still declared.

Any user connected to the database can drop a BLOB filter.

Example

Deleting a BLOB filter.

DROP FILTER DESC_FILTER;

See also

DECLARE FILTER