MySQL索引几乎没有加快简单查询

Question

I have this table, that contains around 80,000,000 rows.

CREATE TABLE `mytable` (
`date` date NOT NULL,
`parameters` mediumint(8) unsigned NOT NULL,
`num` tinyint(3) unsigned NOT NULL,
`val1` int(11) NOT NULL,
`val2` int(10) NOT NULL,
`active` tinyint(3) unsigned NOT NULL,
`ref` int(10) unsigned NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`ref`) USING BTREE,
KEY `parameters` (`parameters`)
) ENGINE=MyISAM AUTO_INCREMENT=79092001 DEFAULT CHARSET=latin1

it's articulated around 2 main columns: "parameters" and "date". there are around 67,000 possible values for "parameters" for each "parameters" there are around 1200 rows, each with a different date. so for each date, there are 67,000 rows. 1200 * 67,000 = 80,400,000.

table size appears as 1.5GB, index size 1.4GB.

now, I want to query the table to retrieve all rows of one "parameters" (actually I want to do it for each parameter, but this is a good start)

SELECT val1 FROM mytable WHERE parameters=1;

the first run gives me results in 8 seconds subsequent runs for different but close values of parameters (2, 3, 4...) are instantaneous

a run for a "far away" value (parameters=1000) gives me results in 8 seconds again.

I did tests running the same query without the index, and got results in 20 seconds, so I guess the index is kicking in as shown by EXPLAIN, but not giving a drastic jump in performances:

+----+-------------+----------+------+---------------+------------+---------+-------+------+-------+
| id | select_type | table    | type | possible_keys | key        | key_len | ref   | rows | Extra |
+----+-------------+----------+------+---------------+------------+---------+-------+------+-------+
|  1 | SIMPLE      | mytable  | ref  | parameters    | parameters | 3       | const | 1097 |       |
+----+-------------+----------+------+---------------+------------+---------+-------+------+-------+

but I'm still baffled by the time for such and easy request (no join, directly on the index).

the server is 2 years-old 2 cpu quad core 2.6GHz running Ubuntu, with 4G of RAM. I've raised the key_buffer parameter to 1G, and have restarted mysql, but noticed no change whatsoever.

should I consider this normal ? or is there something I'm doing wrong ? I get the feeling with the right config the request should be almost immediate.

Answer 1

Try using a covering index, ie create an index that includes both of the columns you need. It won't need the second disk I/O to fetch the values from the main table since the data's right there in the index.