MySQL查询使用filesort和临时

Question

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)

Answer 1

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.

Answer 2

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。