drop table if exists t;

CREATE TABLE `t` (

`id` bigint(20) unsigned NOT NULL auto_increment,

`d` date NOT NULL, 

`a` bigint(20) unsigned NOT NULL,

`b` smallint(5) unsigned DEFAULT NULL,

PRIMARY KEY (`id`,`d`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_cs STATS_SAMPLE_PAGES=200

/*!50500 PARTITION BY RANGE COLUMNS(d)

(PARTITION p20170913 VALUES LESS THAN ('2017-09-14') ENGINE = InnoDB,

PARTITION p20170914 VALUES LESS THAN ('2017-09-15') ENGINE = InnoDB,

PARTITION p20170915 VALUES LESS THAN ('2017-09-16') ENGINE = InnoDB,

PARTITION p20170916 VALUES LESS THAN ('2017-09-17') ENGINE = InnoDB,

PARTITION p20170917 VALUES LESS THAN ('2017-09-18') ENGINE = InnoDB,

PARTITION p99991231 VALUES LESS THAN (MAXVALUE) ENGINE = InnoDB) */;

insert into t(d,a,b) values ('2017-09-15',rand()*10000,rand()*10);

insert into t(d,a,b) values ('2017-09-15',rand()*10000,rand()*10);

replace into t(d,a,b) select '2017-09-15',rand()*10000,rand()*10 from t t1, t t2, t t3, t t4, t t5, t t6, t t7, t t8, t t9, t t10, t t11, t t12, t t13, t t14;

MariaDB [db1]> select count(*) from t where d ='2017-09-15';

+----------+

| count(*) |

+----------+

|    16386 |

+----------+

1 row in set (0.00 sec)

ALTER TABLE t CHANGE b c smallint(5) unsigned , ADD KEY idx_d_a (d, a);

analyze table t;

select count(*) from t where d ='2017-09-15';

select count(*) from t force index(primary) where d ='2017-09-15';

MariaDB [db1]> select count(*) from t where d ='2017-09-15';

+----------+

| count(*) |

+----------+

|        0 |

+----------+

1 row in set (0.00 sec)

MariaDB [db1]> select count(*) from t force index(primary) where d ='2017-09-15';

+----------+

| count(*) |

+----------+

|    16386 |

+----------+

1 row in set (0.00 sec)

MariaDB [db1]> alter table t engine = InnoDB;

Query OK, 0 rows affected (0.24 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [db1]> select count(*) from t where d ='2017-09-15';

+----------+

| count(*) |

+----------+

|    16386 |

+----------+

1 row in set (0.00 sec)