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index merge union not applied
Posted by: Pernot Christian
Date: April 15, 2008 09:40AM

Hi,

I have a index problem, it seems to me that an index merge union should occure.

here is the data:

2 tables: object and object_link

CREATE TABLE object (
idobject int(10) unsigned NOT NULL auto_increment,
idobject_type int(10) unsigned NOT NULL,
idobject_parent int(10) unsigned NOT NULL,
`code` varchar(255) default NULL,
PRIMARY KEY (idobject),
KEY object_index_code (`code`),
KEY object_index_idobject_parent (idobject_parent),
KEY idobject_type (idobject_type),
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

CREATE TABLE object_link (
idobject int(10) unsigned NOT NULL,
idobject_linked int(10) unsigned NOT NULL,
idobject_link_type int(10) unsigned NOT NULL,
number int(11) NOT NULL default '1',
PRIMARY KEY (idobject,idobject_linked,idobject_link_type),
KEY idobject_link (idobject_linked,idobject_link_type),
KEY idobject_target (idobject,idobject_link_type),
KEY idobject_linked (idobject_linked)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;


query :

SELECT
t1.*,
t2.*
FROM
object AS t1
LEFT JOIN object_link AS OL2
ON (OL2.idobject = t1.idobject AND OL2.idobject_link_type = 13 )
LEFT JOIN object AS t2
ON (t2.idobject_parent = t1.idobject OR t2.idobject = OL2.idobject_linked )
WHERE
t1.idobject_parent = 3371

it works but is slow.

here is the EXPLAIN :
+----+-------------+-------+------+--------------------------------------+------------------------------+---------+-------------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+--------------------------------------+------------------------------+---------+-------------+-------+-------------+
| 1 | SIMPLE | t1 | ref | object_index_idobject_parent | object_index_idobject_parent | 4 | const | 20 | |
| 1 | SIMPLE | OL2 | ref | PRIMARY,idobject_target | PRIMARY | 4 | t1.idobject | 1 | Using index |
| 1 | SIMPLE | t2 | ALL | PRIMARY,object_index_idobject_parent | NULL | NULL | NULL | 89059 | |
+----+-------------+-------+------+--------------------------------------+------------------------------+---------+-------------+-------+-------------+

I can't figure out why it doesn't use a index merge for the third one.
I'm using mysql 5.0.45

Any help would be appreciated.



Edited 1 time(s). Last edit at 04/15/2008 09:42AM by Pernot Christian.

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index merge union not applied
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