Sign Up to our social questions and Answers Engine to ask questions, answer people’s questions, and connect with other people.
Login to our social questions & Answers Engine to ask questions answer people’s questions & connect with other people.
Lost your password? Please enter your email address. You will receive a link and will create a new password via email.
Please briefly explain why you feel this question should be reported.
Please briefly explain why you feel this answer should be reported.
Please briefly explain why you feel this user should be reported.
For example, if I have a table with a city and a state column, what is the best way to use the index?
Obviously city will have the highest cardinality, so should I put that column first in the index, should I put state or doesn’t it matter much?
MySQL composite index lookups must take place in the order in which the columns are defined within the index. Since you want MySQL to be able to discriminate between records by performing as few comparisons as possible, with all other things being equal you will benefit most from from a composite index in which the columns are ordered from highest- to lowest-cardinality.
That is, assuming comparisons must eventually be performed against the highest cardinality column in order to discriminate records, why force comparisons to take place first against the lowest cardinality column when ultimately that may be unnecessary?