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DECLARE FILTER

Available in: DSQL, ESQL

Changed in: 2.0

Description

Makes a blob filter known to the database.

Syntax''' _ 

DECLARE FILTER filtername
   INPUT_TYPE <sub_type> OUTPUT_TYPE <subtype>
   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
  • In Firebird 2 and up, no two blob filters in a database may have the same combination of input and output type. Declaring a filter with an already existing input-output type combination will fail. Restoring pre-2.0 databases that contain such “duplicate” filters will also fail.
  • The possibility to indicate the blob types with their mnemonics instead of numbers was added in Firebird 2. The binary mnemonic for subtype 0 was also added in Firebird 2. The predefined mnemonics are case-insensitive.

Example 

declare filter Funnel
   input_type blr output_type text
   entry_point 'blr2asc' module_name 'myfilterlib'

User-defined mnemonics: If you want to define mnemonics for your own blob subtypes, you can add them to the RDB$TYPES system table as shown below. Once committed, the mnemonics can be used in subsequent filter declarations. 

insert into rdb$types (rdb$field_name, rdb$type, rdb$type_name)
  values ('RDB$FIELD_SUB_TYPE', -33, 'MIDI')

The value for rdb$field_name must always be 'RDB$FIELD_SUB_TYPE'. If you define your mnemonics in all-uppercase, you can use them case-insensitively and unquoted in your filter declarations.