I am using a simple MySQL query, but the performance is realy bad because of using ORDER BY. I can't figure out why MySQL is using filesort and temporary.
My query is:
EXPLAIN
SELECT * FROM Events
INNER JOIN EventLogFiles ON ServerID = 42
AND Events.LogFileID = EventLogFiles.LogFileID
ORDER BY ReportID DESC , TimeWritten DESC
LIMIT 100
This is the output of EXPLAIN:
Table Events structure
Table Events indexes
Table EventLogFiles structure
Table EventLogFiles indexes
UPDATE:
I tried to create two new indexes, but both still force MySQL to use filesort and temporary.
ALTER TABLE Events ADD INDEX `ReportID_TimeWritten_ServerID_LogFileID` ( ReportID DESC, TimeWritten DESC, ServerID, LogFileID)
ALTER TABLE Events ADD INDEX `ServerID_LogFileID_ReportID_TimeWritten` ( ServerID, LogFileID, ReportID DESC, TimeWritten DESC)
In order to utilize the index for both selection and sorting, you need to have all of the following columns in a multi-column index on Events: (ServerID, LogFileID, ReportID, TimeWritten)
.
Currently, MySQL cannot utilize the existing multi-column index because it doesn't include LogFileID
, which is in your ON
clause.
If you ever have problems where MySQL is selecting from EventLogFiles first, you can change the INNER JOIN
to a STRAIGHT JOIN
to ensure that MySQL always elects from Events first, using your index.
为了在没有临时表和文件排序的情况下工作,您必须创建索引(ServerID, LogFileID, ReportID)
并且TimeWritten
必须像您在ReportID
使用的auto_increment
一样顺序,因此制作ORDER BY ReportID(PK - Order Physical)
您必须删除filesort。
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