The confusion over global and session status

I was trying to demonstrate to a client how to monitor queries that generate internal temporary tables. With an EXPLAIN plan you see ‘Creating temporary’. Within MySQL you can use the SHOW STATUS to look at queries that create temporary tables.

There is the issue that the act of monitoring impacts the results, SHOW STATUS actually creates a temporary table. You can see in this example.

mysql> select version();
+-----------------+
| version()       |
+-----------------+
| 5.1.31-1ubuntu2 |
+-----------------+
1 row in set (0.00 sec)

mysql> show global status like 'created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 48    |
| Created_tmp_files       | 5     |
| Created_tmp_tables      | 155   |
+-------------------------+-------+
3 rows in set (0.00 sec)

mysql> show global status like 'created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 48    |
| Created_tmp_files       | 5     |
| Created_tmp_tables      | 156   |
+-------------------------+-------+
3 rows in set (0.00 sec)

What has perplexed me in the past, and I can’t explain is that SHOW SESSION STATUS for this example does not increment. It’s confusing to tell a client to use SHOW SESSION STATUS for SQL statements, but the behavior is different with SHOW GLOBAL STATUS. For example, no increment.

mysql> show session status like 'created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0     |
| Created_tmp_files       | 5     |
| Created_tmp_tables      | 2     |
+-------------------------+-------+
3 rows in set (0.00 sec)

mysql> show session status like 'created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0     |
| Created_tmp_files       | 5     |
| Created_tmp_tables      | 2     |
+-------------------------+-------+
3 rows in set (0.00 sec)

Let’s look at a query that creates a temporary table.

mysql> explain select t1.* from t1,t2 where t1.c1 = t2.c2 order by t2.c2, t1.c1;
+----+-------------+-------+------+---------------+------+---------+------+------+---------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                           |
+----+-------------+-------+------+---------------+------+---------+------+------+---------------------------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using temporary; Using filesort |
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using join buffer  |
+----+-------------+-------+------+---------------+------+---------+------+------+---------------------------------+
2 rows in set (0.03 sec)

If we use session status we get an increment of 1.

mysql> show session status like 'created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0     |
| Created_tmp_files       | 5     |
| Created_tmp_tables      | 2     |
+-------------------------+-------+
3 rows in set (0.00 sec)

mysql> show session status like 'created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0     |
| Created_tmp_files       | 5     |
| Created_tmp_tables      | 2     |
+-------------------------+-------+
3 rows in set (0.00 sec)

mysql> select SQL_NO_CACHE t1.* from t1,t2 where t1.c1 = t2.c2 order by t2.c2, t1.c1;
Empty set (0.00 sec)

mysql> show session status like 'created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0     |
| Created_tmp_files       | 5     |
| Created_tmp_tables      | 3     |
+-------------------------+-------+
3 rows in set (0.00 sec)

If we use global status, in this case it’s and idle server so I know there is no other activity, however in a real world situation that isn’t possible.


mysql> show global status like 'created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 48    |
| Created_tmp_files       | 5     |
| Created_tmp_tables      | 171   |
+-------------------------+-------+
3 rows in set (0.00 sec)

mysql> select SQL_NO_CACHE t1.* from t1,t2 where t1.c1 = t2.c2 order by t2.c2, t1.c1;
Empty set (0.00 sec)

mysql> show global status like 'created_tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 48    |
| Created_tmp_files       | 5     |
| Created_tmp_tables      | 173   |
+-------------------------+-------+
3 rows in set (0.00 sec)

2 Responses to “The confusion over global and session status”

  1. Mark Leith says:

    Hey Ronald,

    Yes, previously they incremented both GLOBAL and SESSION outputs – they were fixed as part of http://bugs.mysql.com/bug.php?id=10210 and http://bugs.mysql.com/bug.php?id=29153.

    It’s not just the tmp tables either, it’s also things like Handler_write, and Handler_read_rnd_next. The reason?

    SHOW (and INFORMATION_SCHEMA) create temporary tables in the background, fill them with rows, and then select * from them to show the output to the user.

    For debugging as you describe, you do not want SESSION based counters to increment on top of what you are trying to debug – whilst at the GLOBAL level you want to know all actions that the server was doing.

    The fix actually makes it slightly worse to discount any monitoring action statistics as well (you can not take a monitoring session’s SESSION STATUS and subtract it from a GLOBAL STATUS for the thread local variables).

    Cheers,

    Mark

  2. Arjen Lentz says:

    What Mark says.
    The microslow patch is more effective in finding tmp table use, and it can distinguish between mem and disk also.
    Diddums for sorts.