I can’t remember the last time I stumbled across a bug in MySQL, however this morning I did.
Querying the following table to locate entities with more than one title attribute (this is a diagnostic query, not used in the application).
CREATE TABLE `entity_attr` ( `entity_id` int(11) NOT NULL default '0', `entity_attr_id` int(11) NOT NULL default '0', `seq_no` int(11) NOT NULL default '0', `lang_id` int(11) NOT NULL default '0', `intval` int(11) default NULL, `strval` text, `created` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP, `import_source_id` int(11) NOT NULL default '0', PRIMARY KEY (`entity_id`,`entity_attr_id`,`seq_no`,`lang_id`), KEY `entity_attr_id` (`entity_attr_id`,`strval`(10)), KEY `entity_attr_id_2` (`entity_attr_id`,`intval`), KEY `k_entity_id` (`entity_id`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8
The query:
> SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1 LIMIT 10; +-----------+-----+ | entity_id | num | +-----------+-----+ | 228896 | 2 | | 246726 | 2 | | 409089 | 2 | | 409091 | 2 | | 409098 | 2 | | 409104 | 2 | | 409105 | 2 | | 409106 | 2 | | 409107 | 2 | | 409108 | 2 | +-----------+-----+ 10 rows in set (2.65 sec)
OK, so there are occurrences of entities with multiple title attributes, what’s the most amount of titles a single entity has?
> SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1 ORDER BY num DESC LIMIT 10; +-----------+--------+ | entity_id | num | +-----------+--------+ | 409109 | 525193 | +-----------+--------+ 1 row in set (0.93 sec)
Only one result? Wow, 525k titles! Really?
> SELECT COUNT(*) FROM entity_attr WHERE entity_id = 409109 AND entity_attr_id = 10; +----------+ | COUNT(*) | +----------+ | 1 | +----------+ 1 row in set (0.00 sec)
No.
Lets take another look, remove the limit.
> SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1 ORDER BY num DESC; +-----------+-----+ | entity_id | num | +-----------+-----+ | 409104 | 2 | ... +-----------+-----+ 107 rows in set (0.00 sec)
And if we bring it back.
> SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1 ORDER BY num DESC LIMIT 107; +-----------+--------+ | entity_id | num | +-----------+--------+ | 409109 | 525193 | +-----------+--------+ 1 row in set (0.83 sec)
Looks like LIMIT is causing the issue here. Looks like 5 is a magic LIMIT number too, changing the entity ID in the result:
Limit 4:
+-----------+--------+ | entity_id | num | +-----------+--------+ | 409109 | 525193 | +-----------+--------+ 1 row in set (0.83 sec)
Limit 5:
+-----------+--------+ | entity_id | num | +-----------+--------+ | 55446 | 525193 | +-----------+--------+ 1 row in set (0.00 sec)
Limit 6:
+-----------+--------+ | entity_id | num | +-----------+--------+ | 409109 | 525193 | +-----------+--------+ 1 row in set (0.82 sec)
So how does the query plan change between these statements?
> EXPLAIN SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1 LIMIT 10; +----+-------------+-------------+-------+---------------------------------+---------+---------+------+--------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------------+-------+---------------------------------+---------+---------+------+--------+--------------------------+ | 1 | SIMPLE | entity_attr | index | entity_attr_id,entity_attr_id_2 | PRIMARY | 16 | NULL | 158569 | Using where; Using index | +----+-------------+-------------+-------+---------------------------------+---------+---------+------+--------+--------------------------+
Using the magic limit of 5:
> EXPLAIN SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1 LIMIT 5; +----+-------------+-------------+-------+---------------------------------+---------+---------+------+-------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------------+-------+---------------------------------+---------+---------+------+-------+--------------------------+ | 1 | SIMPLE | entity_attr | index | entity_attr_id,entity_attr_id_2 | PRIMARY | 16 | NULL | 79221 | Using where; Using index | +----+-------------+-------------+-------+---------------------------------+---------+---------+------+-------+--------------------------+
Removing the limit (where we get correct results):
> EXPLAIN SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1; +----+-------------+-------------+------+---------------------------------+------------------+---------+-------+--------+----------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------------+------+---------------------------------+------------------+---------+-------+--------+----------------------------------------------+ | 1 | SIMPLE | entity_attr | ref | entity_attr_id,entity_attr_id_2 | entity_attr_id_2 | 4 | const | 453654 | Using where; Using temporary; Using filesort | +----+-------------+-------------+------+---------------------------------+------------------+---------+-------+--------+----------------------------------------------+
Could there be an issue using the primary key? Lets rebuild the indexes on this table.
> REPAIR TABLE entity_attr QUICK; +------------------+--------+----------+----------+ | Table | Op | Msg_type | Msg_text | +------------------+--------+----------+----------+ | muco.entity_attr | repair | status | OK | +------------------+--------+----------+----------+
Re-run our query:
> SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1 ORDER BY num DESC LIMIT 5; +-----------+--------+ | entity_id | num | +-----------+--------+ | 409101 | 525193 | +-----------+--------+
No joy. Lets simplify slightly:
> SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1 LIMIT 5; +-----------+-----+ | entity_id | num | +-----------+-----+ | 228896 | 2 | | 246726 | 2 | | 409089 | 2 | | 409091 | 2 | | 409098 | 2 | +-----------+-----+
Looks like we’re good if we strip the ORDER BY clause. Are we still relying soley on an index to find rows?
> EXPLAIN SELECT entity_id, COUNT(*) AS num FROM entity_attr WHERE entity_attr_id = 10 GROUP BY entity_id HAVING num > 1 LIMIT 10; +----+-------------+-------------+-------+---------------------------------+---------+---------+------+--------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------------+-------+---------------------------------+---------+---------+------+--------+--------------------------+ | 1 | SIMPLE | entity_attr | index | entity_attr_id,entity_attr_id_2 | PRIMARY | 16 | NULL | 158569 | Using where; Using index | +----+-------------+-------------+-------+---------------------------------+---------+---------+------+--------+--------------------------+
Yup.
So a mix of GROUP, HAVING, ORDER BY with LIMIT doesn’t play well on this version of MySQL.
This is bug wasn’t present on our production servers (running 5.0.x). This issue is exhibited from:
mysql Ver 14.14 Distrib 5.1.37, for debian-linux-gnu (i486) using EditLine wrapper Server version: 5.1.37-1ubuntu5.1-log (Ubuntu)
I’ve not tested this on later releases of the 5.1.x branch. I need a machine I can dirty with two MySQL installs and (on Ubuntu) need to compile from source for the latest (at the time of writing) 5.1.46 release.
UPDATE: I installed Ubuntu 10.04 tonight (April 30th) on a VM and tested this query, the problem persists in Lucid Lynx (version 5.1.41-3ubuntu12).
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