MySQL 二级索引会不会自动补齐主键

开篇：一直对Mysql 二级索引是否自动加入主键问题有怀疑，今天又时间就5.5和5.6分析了一把：

mysql> select version();
+------------+
| version()  |
+------------+
| 5.6.16-log |
mysql> create table t9(
-> id int not null ,
-> a int ,
-> b int,
-> c int,
-> primary key(id),
-> key ab_idx(a,b)
-> )engine=innodb;
Query OK, 0 rows affected (0.00 sec)
show variables like '%optimizer_swit%';
....  use_index_extensions=off
mysql> alter table t9 drop primary key ;
Query OK, 16 rows affected (0.01 sec)
Records: 16  Duplicates: 0  Warnings: 0

mysql> alter table t9 add primary key(id,id2);
mysql> desc select * from t9 where a=2 and b=1 order by id;
+----+-------------+-------+------+---------------+--------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key    | key_len | ref         | rows | Extra       |
+----+-------------+-------+------+---------------+--------+---------+-------------+------+-------------+
|  1 | SIMPLE      | t9    | ref  | ab_idx        | ab_idx | 10      | const,const |    3 | Using where |
+----+-------------+-------+------+---------------+--------+---------+-------------+------+-------------+
1 row in set (0.00 sec)

mysql> desc select * from t9 where a=2 and b=1 order by id,id2;
+----+-------------+-------+------+---------------+--------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key    | key_len | ref         | rows | Extra       |
+----+-------------+-------+------+---------------+--------+---------+-------------+------+-------------+
|  1 | SIMPLE      | t9    | ref  | ab_idx        | ab_idx | 10      | const,const |    3 | Using where |
+----+-------------+-------+------+---------------+--------+---------+-------------+------+-------------+
* 发现是会自动补齐
mysql> select version();
+------------+
| version()  |
+------------+
| 5.5.36-log |
mysql> CREATE TABLE t01 (
->   a char(32) not null,
->   b char(32) not null,
->   c char(32) not null,
->   d char(32) not null,
->   PRIMARY KEY (a,b),
->    KEY idx2 (d,b)
-> ) Engine=InnoDB;
mysql> explain select * from t01 where d='w' and b='g' order by a;
+----+-------------+-------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref         | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+-------------+------+-------------+
|  1 | SIMPLE      | t01   | ref  | idx2          | idx2 | 192     | const,const |    3 | Using where |
+----+-------------+-------+------+---------------+------+---------+-------------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from t01 where d='w' and b='g' order by a,b;
+----+-------------+-------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref         | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+-------------+------+-------------+
|  1 | SIMPLE      | t01   | ref  | idx2          | idx2 | 192     | const,const |    3 | Using where |
+----+-------------+-------+------+---------------+------+---------+-------------+------+-------------+

MySQL 5.6.16 源码 storage/innobase/dict/dict0dict.cc

/*******************************************************************//**
Builds the internal dictionary cache representation for a clustered
index, containing also system fields not defined by the user.
@return	own: the internal representation of the clustered index */
static
dict_index_t*
dict_index_build_internal_clust(
/*============================*/
const dict_table_t*	table,	/*!< in: table */
dict_index_t*		index)	/*!< in: user representation of
a clustered index */
{
dict_index_t*	new_index;
dict_field_t*	field;
ulint		trx_id_pos;
ulint		i;
ibool*		indexed;

ut_ad(table && index);
ut_ad(dict_index_is_clust(index));
ut_ad(mutex_own(&(dict_sys->mutex)));
ut_ad(table->magic_n == DICT_TABLE_MAGIC_N);

/* Create a new index object with certainly enough fields */
new_index = dict_mem_index_create(table->name,
index->name, table->space,
index->type,
index->n_fields + table->n_cols);
/* Remember the table columns already contained in new_index */
indexed = static_cast<ibool*>(
mem_zalloc(table->n_cols * sizeof *indexed));

/* Mark the table columns already contained in new_index */
for (i = 0; i < new_index->n_def; i++) {

field = dict_index_get_nth_field(new_index, i);

/* If there is only a prefix of the column in the index
field, do not mark the column as contained in the index */

if (field->prefix_len == 0) {

indexed[field->col->ind] = TRUE;
}
}

总结：
从源代码看出在引擎层是做了主动补齐主键到二级索引的最后面，但是server层并不知道主键补齐到后面，mysql server层不一定动能自动识别二级索引后面的主键列，强烈建议创建二级索引的时候加上主键列。

我上面的演示实例M ySQL server层是自动自动识别了二级索引后的主键列，看运气吧，目前还没遇到过不识别的。