今天在查询一个表行数的时候，发现count(1)和count(*)执行效率居然是一样的。这跟Oracle还是有区别的。遂查看两种方式的执行计划：
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mysql> select count(1) from customer; +----------+ | count(1) | +----------+ | 150000 | +----------+ 1 row in set (0.03 sec) mysql> flush tables; Query OK, 0 rows affected (0.00 sec) mysql> select count(*) from customer; +----------+ | count(*) | +----------+ | 150000 | +----------+ 1 row in set (0.03 sec)
查看执行计划：
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mysql> explain select count(1) from customer; +----+-------------+----------+-------+---------------+---------------+---------+------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+---------------+---------+------+--------+-------------+ | 1 | SIMPLE | customer | index | NULL | i_c_nationkey | 5 | NULL | 151191 | Using index | +----+-------------+----------+-------+---------------+---------------+---------+------+--------+-------------+ 1 row in set (0.00 sec) mysql> explain select count(*) from customer; +----+-------------+----------+-------+---------------+---------------+---------+------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+---------------+---------+------+--------+-------------+ | 1 | SIMPLE | customer | index | NULL | i_c_nationkey | 5 | NULL | 151191 | Using index | +----+-------------+----------+-------+---------------+---------------+---------+------+--------+-------------+ 1 row in set (0.00 sec) mysql> show index from customer; +----------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +----------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | customer | 0 | PRIMARY | 1 | c_custkey | A | 150525 | NULL | NULL | | BTREE | | | | customer | 1 | i_c_nationkey | 1 | c_nationkey | A | 47 | NULL | NULL | YES | BTREE | | | +----------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 2 rows in set (0.08 sec)
发现不管是count(1)或count(*)都是走的i_c_nationkey这个索引。平时我们检索数据的时候肯定是主键索引效率高，那么我们强制主键索引来看看：
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mysql> select count(*) from customer force index(PRIMARY); +----------+ | count(*) | +----------+ | 150000 | +----------+ 1 row in set (0.68 sec) mysql> explain select count(*) from customer force index(PRIMARY); +----+-------------+----------+-------+---------------+---------+---------+------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+---------+---------+------+--------+-------------+ | 1 | SIMPLE | customer | index | NULL | PRIMARY | 4 | NULL | 150525 | Using index | +----+-------------+----------+-------+---------------+---------+---------+------+---