Artur, no worries, I'm not writing any code ;-)
I did the following:
CREATE TYPE trgm_match AS (match TEXT, threshold NUMERIC);
CREATE OR REPLACE FUNCTION trgm_check_match (string TEXT, match trgm_match)
RETURNS bool
AS 'SELECT match.match <-> string <= 1 - match.threshold'
LANGUAGE SQL;
CREATE OPERATOR %(leftarg = text, rightarg = trgm_match, procedure=trgm_check_match);
This allows me to write:
SELECT ('Warsaw' % row('Warsw', 0.3)::trgm_match);
I'm not sure how to make this operator use an index. It seems I need to create an operator class but I'm not sure how. This is how pg_trgm creates its operator class:
-- create the operator class for gist
CREATE OPERATOR CLASS gist_trgm_ops
FOR TYPE text USING gist
AS
OPERATOR 1 % (text, text),
FUNCTION 1 gtrgm_consistent (internal, text, smallint, oid, internal),
FUNCTION 2 gtrgm_union (internal, internal),
FUNCTION 3 gtrgm_compress (internal),
FUNCTION 4 gtrgm_decompress (internal),
FUNCTION 5 gtrgm_penalty (internal, internal, internal),
FUNCTION 6 gtrgm_picksplit (internal, internal),
FUNCTION 7 gtrgm_same (gtrgm, gtrgm, internal),
STORAGE gtrgm;
Should my operator class mimic the one above?