create table tb_tariff(
 id integer,
 id_category integer,
 st_date date,
 en_date date,
 ...
);

ALTER TABLE TB_TARIFF ADD CONSTRAINT PK_TB_TARIFF PRIMARY KEY (ID);
CREATE INDEX TB_TARIFF_IDX1 ON TB_TARIFF (ID_CATEGORY, ST_DATE, EN_DATE);

param_category =  1 ;
param_date = '01-feb-2004';
select t.*
from tb_tariff t
where (t.id_category = :param_category)
   and (:param_date between t.st_date and t.en_date)

результат - одна строка c st_date = '01-jan-2004' и en_date = '30-apr-2004'

CREATE INDEX TB_TARIFF_IDX2 ON TB_TARIFF (ID_CATEGORY, ST_DATE);
CREATE INDEX TB_TARIFF_IDX3 ON TB_TARIFF (ID_CATEGORY, EN_DATE);

param_category =  1 ;
param_date = '01-feb-2004';

select t.*
from tb_tariff t
where (t.id_abon_category = :param_category)
   and (:param_date >= t.st_date)
   and (t.id_abon_category = :param_category)
   and (:param_date <= t.en_date)

 /* без between`а — вот так тоже 35 чтений */ 
where (t.id_category = :param_category)
   and (:param_date >= t.st_date)
   and (:param_date <= t.en_date)

 /* а вот именно так — только 1-но чтение */ 
where (t.id_category = :param_category)  /* для одного индекса */ 
   and (:param_date >= t.st_date)
   and (t.id_category = :param_category)  /* для другого индекса */ 
   and (:param_date <= t.en_date)