举个简单的例子：
CREATE TABLE `sample` (
`i` int(10) unsigned NOT NULL auto_increment,
`j` varchar(255) default NULL,
PRIMARY KEY (`i`),
KEY `j` (`j`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 mysql> show table status like "sample" \G;
*************************** 1. row ***************************
Name: sample
Engine: InnoDB
Version: 10
Row_format: Compact
Rows: 1638757
Avg_row_length: 61
Data_length: 100253696
Max_data_length: 0
Index_length: 128974848
Data_free: 0
Auto_increment: 1638401
Create_time: 2006-07-12 07:31:51
Update_time: NULL
Check_time: NULL
Collation: utf8_general_ci
Checksum: NULL
Create_options:
Comment: InnoDB free: 1591296 kB
1 row in set (0.27 sec)
这个表有1638400条记录，如果通过加只读锁看看：
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select count(i) from sample lock in share mode;
+----------+
| count(i) |
+----------+
| 1638400 |
+----------+
1 row in set (7.02 sec)
show engine innodb status看下：
History list length 5
Total number of lock structs in row lock hash table 6092
LIST OF TRANSACTIONS FOR EACH SESSION:
—TRANSACTION 0 42306982, ACTIVE 89 sec, process no 10099, OS thread id 1878960
6093 lock struct(s), heap size 503104
有503104字节用来锁1638400条记录，每行小于3bits,那么互斥锁占用多少内存呢？
mysql> begin;
Query OK, 0 rows affected (0.00 sec) mysql> select count(i) from sample for update;
+----------+
| count(i) |
+----------+
| 1638400 |
+----------+
1 row in set (8.60 sec)
show engine innodb status的结果是：
History list length 5
Total number of lock structs in row lock hash table 6092
LIST OF TRANSACTIONS FOR EACH SESSION:
—TRANSACTION 0 42306989, ACTIVE 195 sec, process no 10099, OS thread id 1878960
6093 lock struct(s), heap size 503104
MySQL thread id 7429, query id 24542 localhost root
和前一个占用的内存一样，模糊匹配呢？
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select count(*) from sample where j like "5%" lock in share mode;
+----------+
| count(*) |
+----------+
| 102216 |
+----------+
1 row in set (4.44 sec)
show engine innodb status显示：
LIST OF TRANSACTIONS FOR EACH SESSION:
—TRANSACTION 0 42306999, ACTIVE 133 sec, process no 10099, OS thread id 1878960
441 lock struct(s), heap size 44352
100w行大概消耗44KB，每行小于4bits。
整个上面来看占用的内存还是很少的。
文章来源：
https://www.percona.com/blog/2006/07/13/how-much-memory-innodb-locks-really-take/