想要优化count(*)，首先得了解清楚，MySQL是如何处理count(*)的？在MySQL不同版本、不同存储引擎中，对于count(*)的处理方式，是存在差异的。
MyISAM使用过MyISAM存储引擎的DBA，应该都有这感觉：不管表有多大，count(*)总是能够秒出结果。这是因为，MyISAM表将count(*)结果记录下来了
For MyISAM tables, COUNT(*) is optimized to return very quickly if the SELECT retrieves from one table, no other columns are retrieved, and there is no WHERE clause.
This optimization only applies to MyISAM tables, because an exact row count is stored for this storage engine and can be accessed very quickly. COUNT(1) is only subject to the same optimization if the first column is defined as NOT NULL.（1）查看执行计划，看不出来这个优化
mysql> show create table sbtest1\G
*************************** 1. row ***************************
Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`k` int(10) unsigned NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=MyISAM AUTO_INCREMENT=10000001 DEFAULT CHARSET=utf8mb4 MAX_ROWS=10000000
1 row in set (0.00 sec)
mysql> explain select count(*) from sbtest1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set, 1 warning (0.00 sec)（2）重启mysql实例(避免buffer pool的干扰)，执行count(*)，秒出结果，查看profile，物理读为0
mysql> set profiling=1;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> select count(*) from sbtest1;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.00 sec)
mysql> show profiles;
+----------+------------+------------------------------+
| Query_ID | Duration | Query |
+----------+------------+------------------------------+
| 1 | 0.00020175 | select count(*) from sbtest1 |
+----------+------------+------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> show profile all for query 1;
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
| Status | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out | Messages_sent | Messages_received | Page_faults_major | Page_faults_minor | Swaps | Source_function | Source_file | Source_line |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
| starting | 0.000108 | 0.000000 | 0.000095 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 17 | 0 | NULL | NULL | NULL |
| checking permissions | 0.000005 | 0.000000 | 0.000005 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | check_access | sql_authorization.cc | 835 |
| Opening tables | 0.000014 | 0.000000 | 0.000014 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | open_tables | sql_base.cc | 5648 |
| init | 0.000017 | 0.000000 | 0.000017 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 3 | 0 | handle_query | sql_select.cc | 121 |
| System lock | 0.000009 | 0.000000 | 0.000009 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_lock_tables | lock.cc | 321 |
| optimizing | 0.000006 | 0.000000 | 0.000005 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | optimize | sql_optimizer.cc | 151 |
| executing | 0.000006 | 0.000000 | 0.000006 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | exec | sql_executor.cc | 119 |
| end | 0.000003 | 0.000000 | 0.000003 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | handle_query | sql_select.cc | 199 |
| query end | 0.000004 | 0.000000 | 0.000004 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_execute_command | sql_parse.cc | 4952 |
| closing tables | 0.000007 | 0.000000 | 0.000007 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_execute_command | sql_parse.cc | 5004 |
| freeing items | 0.000014 | 0.000000 | 0.000013 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_parse | sql_parse.cc | 5578 |
| cleaning up | 0.000010 | 0.000000 | 0.000010 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | dispatch_command | sql_parse.cc | 1864 |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
12 rows in set, 1 warning (0.00 sec)InnoDB(5.7.18之前版本)在MySQL 5.7.18之前版本，MySQL是通过扫描聚集索引(即全表扫描)，来获取count(*)的结果
Prior to MySQL 5.7.18, InnoDB processes SELECT COUNT(*) statements by scanning the clustered index.（1）查看执行计划，走的是全表扫描
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.16-log |
+------------+
1 row in set (0.00 sec)
mysql> show create table sbtest1\G
*************************** 1. row ***************************
Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`k` int(10) unsigned NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=10000001 DEFAULT CHARSET=utf8mb4 MAX_ROWS=1000000
1 row in set (0.00 sec)
mysql> explain select count(*) from sbtest1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set, 1 warning (0.00 sec)（2）重启mysql实例(避免buffer pool的干扰)，执行count(*)，查看profile，耗时105s左右，物理读4446672
mysql> set profiling=1;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> select count(*) from sbtest1;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (1 min 45.81 sec)
mysql> show profiles;
+----------+--------------+------------------------------+
| Query_ID | Duration | Query |
+----------+--------------+------------------------------+
| 1 | 105.81688950 | select count(*) from sbtest1 |
+----------+--------------+------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> show profile all for query 1;
+----------------------+------------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
| Status | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out | Messages_sent | Messages_received | Page_faults_major | Page_faults_minor | Swaps | Source_function | Source_file | Source_line |
+----------------------+------------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
| starting | 0.000093 | 0.000037 | 0.000043 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 17 | 0 | NULL | NULL | NULL |
| checking permissions | 0.000005 | 0.000002 | 0.000003 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | check_access | sql_authorization.cc | 835 |
| Opening tables | 0.000013 | 0.000006 | 0.000007 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | open_tables | sql_base.cc | 5648 |
| init | 0.000018 | 0.000008 | 0.000009 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 3 | 0 | handle_query | sql_select.cc | 121 |
| System lock | 0.000007 | 0.000003 | 0.000004 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_lock_tables | lock.cc | 321 |
| optimizing | 105.816679 | 7.909304 | 1.584205 | 105509 | 30 | 4446672 | 0 | 0 | 0 | 0 | 555848 | 0 | optimize | sql_optimizer.cc | 151 |
| executing | 0.000019 | 0.000011 | 0.000002 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | exec | sql_executor.cc | 119 |
| end | 0.000003 | 0.000002 | 0.000001 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | handle_query | sql_select.cc | 199 |
| query end | 0.000009 | 0.000008 | 0.000002 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_execute_command | sql_parse.cc | 4952 |
| closing tables | 0.000010 | 0.000008 | 0.000002 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_execute_command | sql_parse.cc | 5004 |
| freeing items | 0.000020 | 0.000016 | 0.000004 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_parse | sql_parse.cc | 5578 |
| cleaning up | 0.000014 | 0.000011 | 0.000003 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | dispatch_command | sql_parse.cc | 1864 |
+----------------------+------------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
12 rows in set, 1 warning (0.00 sec)InnoDB(5.7.18之后版本)从MySQL 5.7.18版本开始，MySQL会尽量选择扫描二级索引，来获取count(*)的结果
As of MySQL 5.7.18, InnoDB processes SELECT COUNT(*) statements by traversing the smallest available secondary index unless an index or optimizer hint directs the optimizer to use a different index. If a secondary index is not present, the clustered index is scanned.（1）查看执行计划，走的是二级索引k_1
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.30-log |
+------------+
1 row in set (0.00 sec)
mysql> show create table sbtest1\G
*************************** 1. row ***************************
Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`k` int(10) unsigned NOT NULL DEFAULT '0',
`c` char(120) NOT NULL DEFAULT '',
`pad` char(60) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=10000001 DEFAULT CHARSET=utf8mb4 MAX_ROWS=1000000
1 row in set (0.00 sec)
mysql> explain select count(*) from sbtest1;
+----+-------------+---------+------------+-------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+------+---------+------+---------+----------+-------------+
| 1 | SIMPLE | sbtest1 | NULL | index | NULL | k_1 | 4 | NULL | 9745977 | 100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)（2）重启mysql实例(避免buffer pool的干扰)，执行count(*)，查看profile，耗时6s左右，物理读269008
mysql> set profiling=1;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> select count(*) from sbtest1;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (5.99 sec)
mysql> show profiles;
+----------+------------+------------------------------+
| Query_ID | Duration | Query |
+----------+------------+------------------------------+
| 1 | 5.99044700 | select count(*) from sbtest1 |
+----------+------------+------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> show profile all for query 1;
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
| Status | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out | Messages_sent | Messages_received | Page_faults_major | Page_faults_minor | Swaps | Source_function | Source_file | Source_line |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
| starting | 0.000081 | 0.000038 | 0.000036 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 5 | 0 | NULL | NULL | NULL |
| checking permissions | 0.000006 | 0.000003 | 0.000003 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | check_access | sql_authorization.cc | 809 |
| Opening tables | 0.000015 | 0.000007 | 0.000007 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | open_tables | sql_base.cc | 5781 |
| init | 0.000018 | 0.000010 | 0.000009 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 4 | 0 | handle_query | sql_select.cc | 128 |
| System lock | 0.000008 | 0.000003 | 0.000004 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_lock_tables | lock.cc | 330 |
| optimizing | 0.000004 | 0.000002 | 0.000001 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | optimize | sql_optimizer.cc | 158 |
| statistics | 0.000012 | 0.000006 | 0.000006 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | optimize | sql_optimizer.cc | 374 |
| preparing | 0.000013 | 0.000007 | 0.000006 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 3 | 0 | optimize | sql_optimizer.cc | 482 |
| executing | 0.000002 | 0.000001 | 0.000001 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | exec | sql_executor.cc | 126 |
| Sending data | 5.990151 | 1.727173 | 0.094130 | 10985 | 0 | 269008 | 0 | 0 | 0 | 0 | 31023 | 0 | exec | sql_executor.cc | 202 |
| end | 0.000013 | 0.000006 | 0.000001 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | handle_query | sql_select.cc | 206 |
| query end | 0.000012 | 0.000010 | 0.000002 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | mysql_execute_command | sql_parse.cc | 4956 |
| closing tables | 0.000008 | 0.000007 | 0.000001 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_execute_command | sql_parse.cc | 5009 |
| freeing items | 0.000022 | 0.000019 | 0.000004 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | mysql_parse | sql_parse.cc | 5622 |
| logging slow query | 0.000069 | 0.000057 | 0.000011 | 0 | 0 | 0 | 8 |
