why not using index

[Raul Carlomagno]

im using mysql 4.1.7 and i have these tables:
Texts (584949 rows)
Properties (543237 rows)
Classifieds (1269543 rows)
Properties_zones (634104 rows)
Zones (331 rows)

tables ddl:
--------------------
CREATE TABLE `Texts` (
`Text_id` int(11) NOT NULL auto_increment,
`Inmobiliaria_id` int(11) default NULL,
`Original` blob,
`Traducido` blob,
PRIMARY KEY (`Text_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_spanish_ci
---------------------
CREATE TABLE `Properties` (
`Propertie_id` int(11) NOT NULL auto_increment,
`Text_id` int(11) default NULL,
`Tipo_Inmueble` varchar(10) collate latin1_spanish_ci default NULL,
`Tipo_Operacion` varchar(6) collate latin1_spanish_ci default NULL,
`Superficie` double default NULL,
`Moneda` char(1) collate latin1_spanish_ci default NULL,
`Valuacion` double default NULL,
`Tipo_Atributo` varchar(10) collate latin1_spanish_ci default NULL,
`Ambientes` varchar(10) collate latin1_spanish_ci default NULL,
`Marca` tinyint(4) default NULL,
PRIMARY KEY (`Propertie_id`),
KEY `Text_id` (`Text_id`),
KEY `Tipo_Inmueble` (`Tipo_Inmueble`),
CONSTRAINT `Propiedades_ibfk_1` FOREIGN KEY (`Text_id`) REFERENCES `Texts` (`Text_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_spanish_ci
---------------------
CREATE TABLE `Classifieds` (
`Classified_Id` int(11) NOT NULL auto_increment,
`Text_id` int(11) default NULL,
`Medio_id` int(11) default NULL,
`classif_date` date default NULL,
PRIMARY KEY (`Avisos_Id`),
KEY `Text_id` (`Text_id`),
KEY `Medio_id` (`Medio_id`),
KEY `clasiff_date` (`clasiff_date`),
CONSTRAINT `Avisos_ibfk_1` FOREIGN KEY (`Text_id`) REFERENCES `Texts` (`Text_id`),
CONSTRAINT `Avisos_ibfk_2` FOREIGN KEY (`Medio_id`) REFERENCES `Medios` (`Medio_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_spanish_ci
-----------------------
CREATE TABLE `Properties_zones` (
`PropertieZone_id` int(11) default NULL,
`Propertie_id` int(11) NOT NULL default '0',
`Zone_id` int(11) NOT NULL default '0',
`Mtch` smallint(5) default NULL,
PRIMARY KEY (`Propertie_id`,`Zone_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_spanish_ci
-----------------------
CREATE TABLE `Zones` (
`Zone_id` int(11) NOT NULL auto_increment,
`Name` varchar(37) collate latin1_spanish_ci default NULL,
`Localidad_id` int(11) default NULL,
`TipoZona_id` smallint(5) default NULL,
PRIMARY KEY (`Zone_id`),
KEY `Localidad_id` (`Localidad_id`),
KEY `TipoZona_id` (`TipoZona_id`),
CONSTRAINT `Zonas_ibfk_1` FOREIGN KEY (`Localidad_id`) REFERENCES `Localidades` (`Localidad_id`),
CONSTRAINT `Zonas_ibfk_2` FOREIGN KEY (`TipoZona_id`) REFERENCES `TipoZona` (`TipoZona_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_spanish_ci

----------------------
here is the problem:

when i do:
explain select count(0) from Classifieds where classif_date between '2004-06-01' and '2004-06-15'
it gives me:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE a range classif_date classif_date 4 (NULL) 155104 Using where; Using index

it does use the index 'classif_date'



but when i do:
explain select count(t.texxt_id) from Texts t inner join Properties p on p.text_id = t.text_id
INNER join Classifieds a on a.text_id = t.text_id
inner join Properties_zones pz on p.propertie_id = pz.propertie_id
inner join Zones z on pz.zone_id = z.zone_id
where
z.zone_id = 23 and a.classif_date between '2004-06-01' and '2004-06-15'

it gives me:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE z const PRIMARY PRIMARY 4 const 1 Using index
1 SIMPLE a range Text_id,classif_date classif_date 4 (NULL) 155104 Using where
1 SIMPLE t eq_ref PRIMARY PRIMARY 4 MYDB.a.Text_id 1 Using index
1 SIMPLE p ref PRIMARY,Text_id Text_id 5 MYDB.t.Text_id 1 Using where; Using index
1 SIMPLE pz eq_ref PRIMARY PRIMARY 8 MYDB.p.Propertie_id,const 1 Using index


the table Classifieds is not using the classifdate index, so mysql has to read all table, why mysql is not using it?