Question about mysql indexes on low to medium cardinality columns

I have a general question about the way that database indexing works, particularly in mysql.开发者_StackOverflow中文版 Let's say I have a table with a million rows with a column "ClientID" that is distributed relatively equally among 30 values. Thus, this column is very low cardinality (30) relative to the primary key (1 million).

Now, I understand that you shouldn't create indexes on low cardinality fields. However, in this case, queries are only ever done with one of the 30 clientIDs. Thus, wouldn't creating an index on ClientID be helpful, as the search space is automatically reduced to 1/30th what it normally would be? Or is my understanding of how the index works flawed?

Thanks

---

What I've learned as a very general rule of thumb from the MySQL-sponsored seminars I've attended is that you gain SOME level of performance benefit in most cases where the cardinality reduces your search space by 80% or more. This seems to hold across just about any hardware setup I've encountered.

This looks like an index you would almost certainly benefit from. Of course, this can quickly break down on a heavy-write table where the index constantly needs to be adjusted.

In other circumstances, the proper answer is really 'it depends', and benchmarking is usually the way to go.

You may also want to check the BENCHMARK() function in mysql - it's not perfect but can help in testing things like this: http://dev.mysql.com/doc/refman/5.1/en/information-functions.html#function_benchmark