MySQL: low cardinality/selectivity columns = how to index?

Question

I need to add indexes to my table (columns) and stumbled across this post:

How many database indexes is too many?

Quote: “Having said that, you can clearly

Answer 1

I'm sorry to say that I do not agree with Mike. Adding an index is meant to limit the amount of full records searches for MySQL, thereby limiting IO which usually is the bottleneck.

This indexing is not free; you pay for it on inserts/updates when the index has to be updated and in the search itself, as it now needs to load the index file (full text index for 300M records is probably not in memory). So it might well be that you get extra IO in stead of limitting it.

I do agree with the statement that a binary variable is best stored as one, a bool or tinyint, as that decreases the length of a row and can thereby limit disk IO, also comparisons on numbers are faster.

If you need speed and you seldom use the disabled records, you may wish to have 2 tables, one for enabled and one for disabled records and move the records when the status changes. As it increases complexity and risk this would be my very last choice of course. Definitely do the move in 1 transaction if you happen to go for it.

It just popped into my head that you can check wether an index is actually used by using the explain statement. That should show you how MySQL is optimizing the query. I don't really know hoe MySQL optimizes queries, but from postgresql I do know that you should explain a query on a database approximately the same (in size and data) as the real database. So if you have a copy on the database, create an index on the table and see wether it's actually used. As I said, I doubt it, but I most definitely don't know everything:)