Mysql performance concern

[hvle]

I have a table name prob.

prob has 2 fields:

- ID: Interger, AutoIncrement

- Code: varchar 20

 

both fields are indexed and table is myISAM.

 

I currently have about 3 millions records in the table. In other word, I have 3 millions codes, these codes are distinct.

 

when I query: select * from prob where code='84l4jf983jf938'; (this code is not existed in database)

 

After about 6 seconds, an empty result is returned.

 

This post is not about error, I have no error. I only concern about the performance: 6 seconds is disasterous!!!

 

I suppose the indexes are store in binary tree, 3 millions records will average up to about depth 13. If it is 13 depth, how can it take 6 seconds to compare 13 20-chars string?

 

Any mysql expert might explain why it should take ridiculously long, and how can I optimize it?