MySQL database optimization How to choose the right column to index

1. In the WHERE clause, GROUP BY clause, ORDER BY clause, the column appearing on the clause;

2. The smaller the index field, the better;

3. The columns with large dispersion are placed in front of the federated Index, for example:

SELECT * FROM payment where staff_id = 2 and customer_id = 236;

is index (sftaff_id, customer_id) good for the query above? Or is index (customer_id, staff_id) good?

Because the customer_id is more discrete, so use the back more appropriate!!

So here's the problem. How to determine the degree of dispersion, you can use SELECT COUNT (Distinct customer_id), COUNT (distinct staff_id) from table name

Whose value is large indicates that some of these columns are more discrete!

MySQL database optimization How to choose the right column to index