Ejemplo 9: indice funcional

CREATE FUNCTION date_mes ... AS '
    SELECT EXTRACT(year FROM $1) ||
        CASE WHEN EXTRACT(month FROM $1) < 10 THEN
            ''0'' || EXTRACT(month FROM $1)
        ELSE EXTRACT(month FROM $1)
        END
    ' LANGUAGE SQL IMMUTABLE;
CREATE INDEX indice_func ON tabla date_mes(expr);

SELECT * FROM tabla WHERE date_mes(expr) = ...
SELECT * FROM tabla WHERE date_mes(expr) BETWEEN ... AND ...