Slow MySQL query | 易学教程

问题

Hey I have a very slow MySQL query. I'm sure all I need to do is add the correct index but all the things I try don't work.

The query is:

SELECT DATE(DateTime) as 'SpeedDate', avg(LoadTime) as 'LoadTime'
FROM SpeedMonitor
GROUP BY Date(DateTime);

The Explain for the query is:

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  SpeedMonitor    ALL                 7259978 Using temporary; Using filesort

And the table structure is:

CREATE TABLE `SpeedMonitor` (
  `SMID` int(10) unsigned NOT NULL auto_increment,
  `DateTime` datetime NOT NULL,
  `LoadTime` double unsigned NOT NULL,
  PRIMARY KEY  (`SMID`)
) ENGINE=InnoDB AUTO_INCREMENT=7258294 DEFAULT CHARSET=latin1;

Any help would be greatly appreciated.

回答1:

You're just asking for two columns in your query, so indexes could/should go there:

DateTime
LoadTime

Another way to speed your query up could be split DateTime field in two: date and time.
This way db can group directly on date field instead of calculating DATE(...).

EDITED:
If you prefer using a trigger, create a new column(DATE) and call it newdate, and try with this (I can't try it now to see if it's correct):

CREATE TRIGGER upd_check BEFORE INSERT ON SpeedMonitor
FOR EACH ROW
BEGIN
  SET NEW.newdate=DATE(NEW.DateTime);
END

EDITED AGAIN:
I've just created a db with the same table speedmonitor filled with about 900,000 records.
Then I run the query SELECT newdate,AVG(LoadTime) loadtime FROM speedmonitor GROUP BY newdate and it took about 100s!!
Removing index on newdate field (and clearing cache using RESET QUERY CACHE and FLUSH TABLES), the same query took 0.6s!!!
Just for comparison: query SELECT DATE(DateTime),AVG(LoadTime) loadtime FROM speedmonitor GROUP BY DATE(DateTime) took 0.9s.
So I suppose that the index on newdate is not good: remove it.
I'm going to add as many records as I can now and test two queries again.

FINAL EDIT:
Removing indexes on newdate and DateTime columns, having 8mln records on speedmonitor table, here are results:

selecting and grouping on newdate column: 7.5s
selecting and grouping on DATE(DateTime) field: 13.7s

I think it's a good speedup.
Time is taken executing query inside mysql command prompt.

回答2:

The problem is that you're using a function in your GROUP BY clause, so MySQL has to evaluate the expression Date(DateTime) on every record before it can group the results. I'd suggest adding a calculated field for Date(DateTime), which you could then index and see if that helps your performance.

回答3:

I hope you'll permit me to point out that before you put a table into production with millions of records you should seriously consider how that data is going to be used and plan accordingly.

What is happening right now is that your query cannot use any indexes and hence scans the entire table building a response. Not the fastest way to work with relatively large tables.

You have some things to consider if you want to get to a better state:

How fast is it collecting data?
How much history do you need?
How granular are your reporting requirements?
Are you able to suspend logging to make table changes?

If the answer is "No" to the last question you could always create a new table/solution and start writing records there... importing in old data if/as needed.

Reporting granularity is important as you could, for example, compress a day's worth of data into 24 records. Load the current day into an index free loading table and then process it the next day into per hour averages. Name each loading table based on the sample date and you can delete old tables as processed.

Of course, hourly may not be fine grained enough.

Depending on your retention needs you might want to consider some type of partitioned storage. This can let you query against subsets of sample data and simply drop or archive old partitions when they are no long current enough to be relevant.

Anyhow, you seem to be on the edge of having some type of massive sampling, reporting and/or monitoring system (particularly if you were reporting on a variety of sites or pages with different characteristics). You may want to put some effort into designing this so it will fit your needs... ;)

来源：https://stackoverflow.com/questions/5642880/slow-mysql-query

标签

mysql

performance

filesort