DECLARE FILTER

`DECLARE FILTER`

Declares a BLOB filter in the current 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

Table 1. `DECLARE FILTER` Statement Parameters
Parameter	Description
filtername	Filter name in the database.The maximum length is 63 characters.It need not be the same name as the name exported from the filter library via `ENTRY_POINT`.
sub_type	`BLOB` subtype
number	`BLOB` subtype number (must be negative)
mnemonic	`BLOB` subtype mnemonic name
function_name	The exported name (entry point) of the function
library_name	The name of the module where the filter is located
user_defined	User-defined `BLOB` subtype mnemonic name

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), or their user-defined name from the RDB$TYPES table.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.

Note

Mnemonic names can be defined for custom BLOB subtypes and inserted manually into the system table RDB$TYPES system table:

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

After the transaction is committed, the mnemonic names can be used in declarations when you create new filters.

The value of the column RDB$FIELD_NAME must always be 'RDB$FIELD_SUB_TYPE'.If a mnemonic names was defined in upper case, they can be used case-insensitively and without quotation marks when a filter is declared, following the rules for other object names.

Warning

In general, the system tables are not writable by users.However, inserting custom types into RDB$TYPES is still possible if the user is an administrator, or has the system privilege CREATE_USER_TYPES.