I have forgotten this :
CREATE OR REPLACE FUNCTION date_to_yearmonth_dec(TIMESTAMP) RETURNS VARCHAR AS'
BEGIN
RETURN extract(year FROM $1) || extract(month FROM $1)-1;
END'LANGUAGE plpgsql IMMUTABLE;
---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match