MySQL innoDB: Long time of query execution

问题

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

回答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.

回答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

回答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.

来源：https://stackoverflow.com/questions/16481049/mysql-innodb-long-time-of-query-execution

标签

mysql

sql

query-optimization

InnoDB