select count(), keys und left() ?
Posted: 2004-09-23 17:09
Code: Select all
mysql> select count(id) from test where left(id,1) = "d";
+---------------+
| count(id) |
+---------------+
| 0 |
+---------------+
1 row in set (1.83 sec)
mysql> explain select count(id) from test where left(id,1) = "d";
+---------+-------+---------------+--------+---------+------+-------+-------------------------+
| table | type | possible_keys | key | key_len | ref | rows | Extra
|
+---------+-------+---------------+--------+---------+------+-------+-------------------------+
| test | index | NULL | id | 256 | NULL | 80352 | where
used; Using index |
+---------+-------+---------------+--------+---------+------+-------+-------------------------+
1 row in set (0.00 sec)
mysql> select count(*) from test;
+----------+
| count(*) |
+----------+
| 80352 |
+----------+
1 row in set (0.00 sec)