Mysql doesn't use my Index correctly - mysql

Question

First sorry, i am french and i don't speak very well english.
I have a rather strange problem with my index.
my table t_bloc (my table contains all the posts)
t_bloc
CREATE TABLE `t_bloc` (
 `id_bloc` int(10) unsigned NOT NULL AUTO_INCREMENT,
 `id_rubrique` int(10) unsigned NOT NULL DEFAULT '0',
 `titre` varchar(100) NOT NULL DEFAULT 'A compléter',
 `contenu` text NOT NULL,
 `titre_page` varchar(100) NOT NULL,
 `desc_courte` varchar(255) NOT NULL,
 `url` varchar(255) NOT NULL,
 `follow_url` tinyint(1) unsigned NOT NULL DEFAULT '1' ,
 `image` varchar(120) NOT NULL,
 `video` varchar(120) NOT NULL,
 `note` tinyint(3) unsigned NOT NULL DEFAULT '0',
 `champopt1` text NOT NULL,
 `champopt2` text NOT NULL,
 `permalien` varchar(100) NOT NULL,
 `en_ligne` tinyint(1) NOT NULL DEFAULT '0',
 `id_utilisateur` int(10) unsigned NOT NULL DEFAULT '1' ,
 `date_crea` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
 `date_modif` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
 `nb_commentaires` smallint(5) unsigned NOT NULL DEFAULT '0',
 `nb_jaimes` int(10) unsigned NOT NULL DEFAULT '0',
 `nb_jaimespas` int(10) unsigned NOT NULL DEFAULT '0',
 `pourcentage_jaimes` smallint(2) unsigned NOT NULL DEFAULT '0' ,
 `niveau_classement` tinyint(1) unsigned NOT NULL DEFAULT '1' ,
 `id_bloc_parent` int(10) unsigned NOT NULL DEFAULT '0' ,
 `id_membre_bloc` int(10) unsigned NOT NULL DEFAULT '0' ,
 `valeur_pts_bloc` smallint(5) unsigned NOT NULL DEFAULT '0' ,
 `commentaires_actifs` tinyint(1) unsigned NOT NULL DEFAULT '1' ,
 PRIMARY KEY (`id_bloc`),
 KEY `date_modif` (`date_modif`),
 KEY `id_bloc_parent` (`id_bloc_parent`),
 KEY `idx_rub_ligne_niv_etc` (`id_rubrique`,`en_ligne`,`niveau_classement`,`note`,`pourcentage_jaimes`,`date_modif`),
 KEY `idx_tri` (`en_ligne`,`niveau_classement`,`note`,`pourcentage_jaimes`,`date_modif`),
 KEY `idx_ligne_membre` (`en_ligne`,`id_membre_bloc`,`id_rubrique`),
 KEY `id_membre_bloc` (`id_membre_bloc`),
 KEY `idx_rub_ligne_date` (`id_rubrique`,`en_ligne`,`date_modif`,`id_bloc`),
 KEY `idx_ligne_date` (`en_ligne`,`date_modif`),
 FULLTEXT KEY `idx_fullindex` (`titre`,`contenu`)
) ENGINE=MyISAM AUTO_INCREMENT=456469 DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC
My table t_taxon_bloc
t_taxon_bloc
CREATE TABLE `t_taxon_bloc` (
 `id_taxon` int(10) unsigned NOT NULL,
 `id_bloc` int(10) unsigned NOT NULL,
 `url_plateforme` varchar(255) NOT NULL,
 `width_flash` smallint(5) unsigned NOT NULL,
 `height_flash` smallint(5) unsigned NOT NULL,
 PRIMARY KEY (`id_taxon`,`id_bloc`),
 KEY `id_bloc` (`id_bloc`),
 KEY `id_taxon` (`id_taxon`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC
I have a problem when I execute this query:
select b.id_bloc FROM t_bloc as b WHERE b.en_ligne = 1 AND EXISTS ( SELECT 1 FROM t_taxon_bloc AS TB WHERE TB.id_bloc=B.id_bloc AND TB.id_taxon= 83) ORDER BY b.en_ligne DESC, b.date_modif DESC LIMIT 0, 20
I get the following explain:
id
select_type
table
type
possible_keys
key
key_len
ref
rows
Extra
1
PRIMARY
b
ref
idx_tri,idx_ligne_membre,idx_ligne_date
idx_ligne_date
1
const
58210
Using where
2
DEPENDENT SUBQUERY
TB
eq_ref
PRIMARY,id_bloc,id_taxon
PRIMARY
8
const,sitajeuxtestbourrage.b.id_bloc
1
Using index
it does not fully use the idx_ligne_date index and rows = all rows in the table
Extra = « using Where »
But if I create the following index idx_ligne_date_idbloc (en_ligne, date_modif, id_bloc) ,
the use of the index is a little better, the application runs a little faster.
id
select_type
table
type
possible_keys
key
key_len
ref
rows
Extra
1
PRIMARY
b
ref
idx_tri,idx_ligne_membre,idx_ligne_date_idbloc
idx_ligne_date_idbloc
1
const
58252
Using where; Using index
2
DEPENDENT SUBQUERY
TB
eq_ref
PRIMARY,id_bloc,id_taxon
PRIMARY
8
const,sitajeuxtestbourrage.b.id_bloc
1
Using index
Extra = « using Where, Using Index »
My questions:
id_bloc does not appear in the where and order by clauses, why am I required to add id_bloc on my multiple index ?
And Why rows = (again) all my table ? And not 20 (LIMIT 0,20)

Answer 1

You seem to have a misunderstanding what "Using Index" means in the Extra column. Have a look at this french explanation: http://use-the-index-luke.com/fr/sql/plans-dexecution/mysql/operations
id_bloc does not appear in the where and order by clauses, why am I required to add id_bloc on my multiple index ?
You are not required to, but doing so allows a so-called index-only scan (which is indicated by the words "Using Index" in Extra). You can learn about index-only scan at this french page: http://use-the-index-luke.com/fr/sql/regrouper-les-donnees/parcours-d-index-couvrants