MySQL 的 EF 查询性能问题

Question

I am using Entity Framework 6 with MySQL 5.7. I am trying to tune queries and evaluate performance problems. There is a particular query showing up in the Slow Query Log. Here is an entry:

# Time: 2021-02-10T17:28:40.313796Z
# User@Host: user @ localhost [::1]  Id:    89
# Query_time: 40.252085  Lock_time: 0.000044 Rows_sent: 1  Rows_examined: 758678
SET timestamp=1612978120;
SELECT
`Project1`.`Id`, 
`Project1`.`Time`, 
`Project1`.`Message`, 
`Project1`.`Device_Id`, 
`Project1`.`Type_Id`
FROM `DeviceEvents` AS `Project1`
 WHERE (`Project1`.`Device_Id` = 2) AND (`Project1`.`Type_Id` = 7)
 ORDER BY 
`Project1`.`Time` DESC LIMIT 1;

As you can see the query is taking over 40 seconds. If I copy this exact query in Workbench and run it, it takes 0.003 seconds. So, I am not sure what EF is doing that is affecting the execution time when it gets to MySQL. It looks like MySQL is examining a lot of rows even though there is an index on Device_Id, Type_Id, and Time.

Answer 1

After posting this I figured out the problem. Even though there were individual indexes on each of the appropriate columns, there was not a composite index. I added a composite index for (Device_Id, Type_Id, Time) and now the query returns almost immediately.

I am not sure why it would run fast in Workbench without the composite index but not through Entity Framework.