Why is Mysql not using a part index?

Question

So I have a table of IP addresses and RIR ranges and so want to link on the first 3 characters of the IP address in a join.

I have created the following index: CREATE INDEX My_range ON ranges ( Range (3))

I then run the following query:

EXPLAIN SELECT * FROM ips JOIN ranges rt ON SUBSTR(IPAddress,1,3) = SUBSTR( Range ,1,3) WHERE IPID = 17054

And I don't see any indexes available on the rt line. What am I missing??

Answer 1

The following quote is from @juergend in comments.

Indexes can only be used on raw column data and not on data that is processed by a function like substr() .

To add on what he said, you may want to create another indexed column to hold the substring'd value and join on that instead.