indexed query, but still searching every row - mysql

Question

I have the following mysql query
select points_for_user from items where user_id = '38415';
explain on the query returns this
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE items index NULL points_for_user_index 2 NULL 1000511 Using index
The problem is, shouldn't the number of rows be FAR less then the number of rows in the table because of the index?
user_id is the primary index, so I tried creating an index on just points_for_user and that still look through every row. An index on user_id AND points_for_user still searches every row.
What am I missing?
Thanks!
CREATE TABLE IF NOT EXISTS `items` (
`capture_id` int(11) NOT NULL AUTO_INCREMENT,
`id` int(11) NOT NULL,
`creator_user_id` bigint(20) NOT NULL DEFAULT '0',
`user_id` int(11) NOT NULL,
`accuracy` int(11) NOT NULL,
`captured_at` timestamp NOT NULL DEFAULT '2011-01-01 06:00:00',
`ip` varchar(30) NOT NULL,
`capture_type_id` smallint(6) NOT NULL DEFAULT '0',
`points` smallint(6) NOT NULL DEFAULT '5',
`points_for_user` smallint(6) NOT NULL DEFAULT '3',
PRIMARY KEY (`capture_id`),
KEY `capture_user` (`capture_id`,`id`,`user_id`),
KEY `user_id` (`user_id`,`id`),
KEY `id` (`id`),
KEY `capture_creator_index` (`capture_id`,`creator_user_id`),
KEY `points_capture_index` (`points_for_user`,`creator_user_id`),
KEY `points_for_user_index` (`points_for_user`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=1008992 ;
select count(*) from items where user_id = '38415'
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE captures ref user_munzee_id user_munzee_id 4 const 81 Using index

Answer 1

the mysql optimizer try to use the best possible index during the query.
In your first query the optimizer is considering points_for_user_index the best choice, in fact the Extra column show the "Using index" status, this means a "Covering index".
The "Covering index" occurs when all fields required for a query (in your case select points_for_user from ... ) are contained in an index, this avoid the access to the full mysql data (.MYD) in favour of the direct index access (.MYI)
First of all you can try to rebuild the index tree analyzing table
ANALYZE TABLE itemes;
Note for very large tables:
ANALYZE TABLE analyzes and stores the key distribution for a table.
During the analysis, the table is locked with a read lock for InnoDB
and MyISAM. This statement works with InnoDB, NDB, and MyISAM tables.
For MyISAM tables, this statement is equivalent to using myisamchk
--analyze.
If "the problem" persist and you want to bypass the optimizer choice you can explicit try to force the usage of an index
EXPLAIN SELECT points_for_user FROM items USE INDEX ( user_id ) WHERE user_id = '38415'
More details: http://dev.mysql.com/doc/refman/5.5/en/index-hints.html
Cristian