Pourquoi MySQL/innoDB n'utilise pas mon bel index à partir de ???

Bonjour,

J'ai ajouté sur une table en innoDB un index qui boost un max un select.
Il est bien utilisé sur une recherche d'une quinzaine de jours (environ) mais pas au delà !!!
Pourquoi ??! Une limitation ? de quoi ?

Comment faire pour levé cette limitation ?
Sinon comment savoir quand utiliser un autre index (moins bon) ?
(MySQL ne choisit pas le bon...)


Voir ci-dessous,
J'ai essayé avec différant intervalle de longueur et de plage de date,
c'est toujours AUX ENVIRONS de 15 jours que l'index idPlus n'est plus pris en compte.
GRRRrrr !

Merci.


PRESENTATION TABLE
------------------


mysql> show columns from Mes;
+--------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+------------------+------+-----+---------+----------------+
| idx | int(10) unsigned | NO | PRI | NULL | auto_increment |
| rep | int(11) | YES | MUL | NULL | |
| ind | tinyint(4) | YES | | NULL | |
| dtA | datetime(3) | YES | | NULL | |
| val | float | YES | | NULL | |
| infoI1 | int(11) | YES | | NULL | |
| infoS1 | varchar(30) | YES | | NULL | |
| dyA | int(10) unsigned | YES | MUL | 0 | |
+--------+------------------+------+-----+---------+----------------+
8 rows in set (0.01 sec)

mysql> show index from Mes;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------
| mes | 0 | PRIMARY | 1 | idx | A | 13097815 | NULL | NULL | | BTREE
| mes | 1 | idMes | 1 | rep | A | 33 | NULL | NULL | YES | BTREE
| mes | 1 | idMes | 2 | ind | A | 33 | NULL | NULL | YES | BTREE
| mes | 1 | idDyA | 1 | dyA | A | 85 | NULL | NULL | YES | BTREE
| mes | 1 | idPlus | 1 | dyA | A | 8242 | NULL | NULL | YES | BTREE
| mes | 1 | idPlus | 2 | rep | A | 15008 | NULL | NULL | YES | BTREE
| mes | 1 | idPlus | 3 | ind | A | 15008 | NULL | NULL | YES | BTREE
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------
7 rows in set (0.00 sec)

mysql> select * from Mes where dtA > '2020-02-01' limit 5;
+----------+------+------+-------------------------+----------+--------+----------------------+------------+
| idx | rep | ind | dtA | val | infoI1 | infoS1 | dyA |
+----------+------+------+-------------------------+----------+--------+----------------------+------------+
| 62405160 | 681 | 1 | 2020-02-01 00:00:00.744 | 0.146489 | 0 | STM: Mesure opacité | 1580511600 |
| 62405161 | 604 | 1 | 2020-02-01 00:00:08.195 | 1.56353 | 0 | LM: Mesure de vent | 1580511600 |
| 62405162 | 609 | 1 | 2020-02-01 00:00:17.395 | 1.44338 | 0 | LM: Mesure de vent | 1580511600 |
| 62405163 | 606 | 1 | 2020-02-01 00:00:19.296 | 1.64104 | 0 | LM: Mesure de vent | 1580511600 |
| 62405164 | 602 | 1 | 2020-02-01 00:00:19.846 | 1.43067 | 0 | LM: Mesure de vent | 1580511600 |
+----------+------+------+-------------------------+----------+--------+----------------------+------------+

mysql> select count(idx) from Mes;
+------------+
| count(idx) |
+------------+
| 13159102 |
+------------+

LE PROBLEME
-----------

mysql> explain select dtA, val FROM Mes USE INDEX(idPlus) WHERE dyA >= unix_timestamp('2020-02-01') AND dyA <= unix_timestamp('2020-02-15') AND rep=0601 AND ind=1;
+----+-------------+-------+------------+-------+---------------+--------+---------+------+---------+----------+----------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+---------+----------+----------------------------------+
| 1 | SIMPLE | Mes | NULL | range | idPlus | idPlus | 12 | NULL | 2159980 | 0.30 | Using index condition; Using MRR |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+---------+----------+----------------------------------+
==> Requête super rapide... (tout est relatif...)

mysql> explain select dtA, val FROM Mes USE INDEX(idPlus) WHERE dyA >= unix_timestamp('2020-02-01') AND dyA <= unix_timestamp('2020-02-16') AND rep=0601 AND ind=1;
+----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
| 1 | SIMPLE | Mes | NULL | ALL | idPlus | NULL | NULL | NULL | 13097815 | 0.05 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+----------+----------+-------------+
==> Requête super lente...