optimization - Order of columns in a multi-column index in MySQL

Question

I'm trying to understand what is better when defining multi-column indexes:

• Putting the most selective column first (higher cardinality, for speed?); or
• Putting the less selective column first (Lower cardinality, for index compression?)

Or maybe it depends if I'm optimizing for speed or space?

Answer 1

The order of the columns should match the order in which the columns are queried later or MySQL will not use them. This is the question you should really think about.

Read more here.

UPDATE:

For your question about cardinality maybe read this. Is this similar to your question? Does it answer it?