MySQL innoDB: Long time of query execution - mysql

Question

I'm having troubles to run this SQL:
I think it's a index problem but I don't know because I dind't make this database and I'm just a simple programmer.
The problem is, that table has 64260 records, so that query gets crazy when executing, I have to stop mysql and run again because the computer get frozen.
Thanks.
EDIT: table Schema
CREATE TABLE IF NOT EXISTS `value_magnitudes` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`value` float DEFAULT NULL,
`magnitude_id` int(11) DEFAULT NULL,
`sdi_belongs_id` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`reading_date` datetime DEFAULT NULL,
`created_at` datetime DEFAULT NULL,
`updated_at` datetime DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=1118402 ;
Query
select * from value_magnitudes
where id in
(
SELECT min(id)
FROM value_magnitudes
WHERE magnitude_id = 234
and date(reading_date) >= '2013-04-01'
group by date(reading_date)
)
EDIT2

Answer 1

First, add an index on (magnitude_id, reading_date):
ALTER TABLE
ADD INDEX magnitude_id__reading_date__IX -- just a name for the index
(magnitude_id, reading_date) ;
Then try this variation:
SELECT vm.*
FROM value_magnitudes AS vm
JOIN
( SELECT MIN(id) AS id
FROM value_magnitudes
WHERE magnitude_id = 234
AND reading_date >= '2013-04-01' -- changed so index is used
GROUP BY DATE(reading_date)
) AS vi
ON vi.id = vm.id ;
The GROUP BY DATE(reading_date) will still need to apply the function to all the selected (thorugh the index) rows and that cannot be improved, unless you follow #jurgen's advice and split the column into date and time columns.

Answer 2

Since you want to get results for every day you need to extract the date from a datetime column with the function date(). That makes indexes useless.
You can split up the reading_date column into reading_date and reading_time. Then you can run the query without a function and indexes will work.
Additionally you can change the query into a join
select *
from value_magnitudes v
inner join
(
SELECT min(id) as id
FROM value_magnitudes
WHERE magnitude_id = 234
and reading_date >= '2013-04-01'
group by reading_date
) x on x.id = v.id

Answer 3

For starters, I would change your query to:
select * from value_magnitudes where id = (
select min(id) from value_magnitudes
where magnitude_id = 234
and DATE(reading_date) >= '2013-04-01'
)
You don't need to use the IN clause when the subquery is only going to return one record.
Then, I would make sure you have an index on magnitude_id and reading_date (probably a two field index) as that's what you are querying against in the subquery. Without that index, you are scanning the table each time.
Also if possible change magnitude_id and reading_date to non null. Null values and indexes are not great fits.