How to prevent a full table scan when doing a simple JOIN? - mysql

Question

I have two tables, TableA and TableB:
CREATE TABLE `TableA` (
`shared_id` int(10) unsigned NOT NULL default '0',
`foo` int(10) unsigned NOT NULL,
PRIMARY KEY (`shared_id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1
CREATE TABLE `TableB` (
`shared_id` int(10) unsigned NOT NULL auto_increment,
`bar` int(10) unsigned NOT NULL,
KEY `shared_id` (`shared_id`)
) ENGINE=MyISAM AUTO_INCREMENT=1001 DEFAULT CHARSET=latin1
Here's my query:
SELECT TableB.bar
FROM TableB, TableA
WHERE TableA.foo = 1000
AND TableA.shared_id = TableB.shared_id;
Here's the problem:
mysql> explain SELECT TableB.bar FROM TableB, TableA WHERE TableA.foo = 1000 AND TableA.shared_id = TableB.shared_id;
+----+-------------+--------------+--------+---------------+---------+---------+------------------------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+--------+---------------+---------+---------+------------------------------------------+------+-------------+
| 1 | SIMPLE | TableB | ALL | shared_id | NULL | NULL | NULL | 1000 | |
| 1 | SIMPLE | TableA | eq_ref | PRIMARY | PRIMARY | 4 | MyDatabase.TableB.shared_id | 1 | Using where |
+----+-------------+--------------+--------+---------------+---------+---------+------------------------------------------+------+-------------+
Is there an index that I can add that will prevent the full table scan of TableB?

Answer 1

Runcible, your query could use some rewriting. You should always specify your JOIN conditions in an ON clause and not in a WHERE.
Your query would become:
SELECT TableB.bar
FROM TableB
JOIN TableA
ON TableB.shared_id = TableA.shared_id
AND TableA.foo = 1000;
Not only do you want to do this:
ALTER TABLE TableB ADD INDEX (shared_id,bar);
You'll want to add an index to A as follows:
ALTER TABLE TableA ADD INDEX (foo, shared_id);
Do this, and provide the EXPLAIN output please.
Also note that by adding an index on (shared_id, bar) you just made your (shared_id) index redundant. Drop it.