MySQL查询-为什么在这种情况下不使用索引？

Question

MySQL query (ver 5.1.61)

SELECT alerts.*,
       devices.user_id
  FROM alerts
       left JOIN devices
          ON alerts.device_id = devices.id
 WHERE    devices.name = 'myval'

There is an index on "alerts.device_id" MySQL in Explain does not use the index, it shows type "ALL" and rows being the full count of rows in the table.

I can't understand why it doesn't use this index. What am I missing?

Thanks!

Answer 1

The index are not getting picked, because you have not used it, neither in the select nor in the where clause.

You need to add an index on device.name

You can do one thing to force the engine to consider the index if it is a primary key and its value if is more than 0, like :

SELECT alerts.*,
       devices.user_id
  FROM alerts
       left JOIN devices
          ON alerts.device_id = devices.id
 WHERE    devices.name = 'myval' and alerts.deviceid>0