Why isn't the rows of an EXPLAIN accurate?

Question

I have the following table:

mysql> select count(*) from employees;  
+----------+  
| count(*) |  
+----------+  
|    10000 |  
+----------+  
1 row in set (0.03 sec)  

I do an EXPLAIN: mysql> explain select last_name from employees order by last_name;

+----+-------------+-----------+------+---------------+------+---------+------+------+----------------+  
| id | select_type | table     | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-----------+------+---------------+------+---------+------+------+----------------+  
|  1 | SIMPLE      | employees | ALL  | NULL          | NULL | NULL    | NULL | 9894 | Using filesort |  
+----+-------------+-----------+------+---------------+------+---------+------+------+----------------+  
1 row in set (0.00 sec)

The rows are 9894. I was expecting 10000.
I do:

mysql> analyze table employees;
+-------------------------------------+---------+----------+----------+  
| Table                               | Op      | Msg_type | Msg_text |  
+-------------------------------------+---------+----------+----------+  
| sql_dummy.employees | analyze | status   | OK       |  
+-------------------------------------+---------+----------+----------+  
1 row in set (0.04 sec)  

and re-run EXPLAIN:

mysql> explain select last_name from employees order by last_name;

+----+-------------+-----------+------+---------------+------+---------+------+-------+----------------+  
| id | select_type | table     | type | possible_keys | key  | key_len | ref  | rows  | Extra          |  
+----+-------------+-----------+------+---------------+------+---------+------+-------+----------------+  
|  1 | SIMPLE      | employees | ALL  | NULL          | NULL | NULL    | NULL | 10031 | Using filesort |  
+----+-------------+-----------+------+---------------+------+---------+------+-------+----------------+  
1 row in set (0.01 sec)  

The rows are now 10031.

Does anyone know why the rows is never 10000? I have noticed this in other cases as well.

Answer 1

You may find this surprising but that's the behavior of InnoDB.

The InnoDB storage engine does cardinality approximation by walking a few levels down the nonleaf BTREE nodes and takes an educated guess.

I wrote about this a long time ago based on mysqlperformanceblog.com

• Jun 21, 2011 : From where does the MySQL Query Optimizer read index statistics?
• Aug 03, 2011 : When should I rebuild indexes?
• Feb 23, 2013 : How does mysql deal with queries that touches myisam and innodb tables?

Perhaps you could try disabling that behavior by running this:

SET GLOBAL innodb_stats_on_metadata = 0;

Setting innodb_stats_on_metadata should stabilize the approximate cardinality and you should get the same cardinality over and over again.