count(1) count(*)

mysql> select 1 from t;
+---+
| 1 |
+---+
| 1 |
| 1 |
| 1 |
| 1 |
+---+
4 rows in set (0.00 sec)

mysql> select count(1) from t;
+----------+
| count(1) |
+----------+
|        4 |
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from t;
+----------+
| count(*) |
+----------+
|        4 |
+----------+
1 row in set (0.00 sec)

mysql> select count(a) from t;
+----------+
| count(a) |
+----------+
|        4 |
+----------+
1 row in set (0.00 sec)

如果表中没有主键，那么count(1)比count(*)快
如果有主键，那么count(主键，联合主键)比count(*)快
如果表中只有一个字段，count(*)最快