normalize_thai_name

postgres=# CREATE TABLE citizen (name text);
CREATE TABLE
postgres=# INSERT INTO citizen VALUES ('เเซม');
INSERT 0 1
postgres=# SELECT name FROM citizen WHERE name = 'แซม';
 name 
------
(0 rows)

postgres=# CREATE OR REPLACE FUNCTION normalize_thai_name(name text) RETURNS text AS $$
  BEGIN
    RETURN REGEXP_REPLACE(name, 'เเ', 'แ', 'g');
  END;
$$ LANGUAGE plpgsql;
CREATE FUNCTION
postgres=# UPDATE citizen SET name = normalize_thai_name(name);
UPDATE 1
postgres=# SELECT name FROM citizen WHERE name = 'แซม'; name 
------
 แซม
(1 row)

postgres=# SELECT name FROM citizen WHERE name = normalize_thai_name('แซม'); name 
------
 แซม
(1 row)