postgres audit log trigger

This triggers will log before and after fields for a table in JSON format.

CREATE TABLE changelog
(
id serial NOT NULL,
ts timestamp DEFAULT NOW(),
table_name text,
operation text,
new_val json,
old_val json
);
CREATE INDEX changelog_ts_table_name_index
ON changelog (ts DESC, table_name ASC);
CREATE OR REPLACE FUNCTION logchange() RETURNS trigger AS
$$
BEGIN
IF TG_OP = 'INSERT'
THEN
INSERT INTO changelog (table_name, operation, new_val)
VALUES (TG_RELNAME, TG_OP, ROW_TO_JSON(NEW));
RETURN NEW;
ELSIF TG_OP = 'UPDATE'
THEN
INSERT INTO changelog (table_name, operation, new_val, old_val)
VALUES (TG_RELNAME, TG_OP, ROW_TO_JSON(NEW), ROW_TO_JSON(OLD));
RETURN NEW;
ELSIF TG_OP = 'DELETE'
THEN
INSERT INTO changelog
(table_name, operation, old_val)
VALUES (TG_RELNAME, TG_OP, ROW_TO_JSON(OLD));
RETURN OLD;
END IF;
END;
$$ LANGUAGE 'plpgsql' SECURITY DEFINER;
CREATE TRIGGER users_changelog_biudt
BEFORE INSERT OR UPDATE OR DELETE
ON users
FOR EACH ROW
EXECUTE PROCEDURE logchange();