• without index MySQL will search through all table
• used for fast search for WHERE column or for JOIN
• do not create indexes in advance, but for particular high load
• create indexes if slow query log is more then 1 second
• create indexes only for frequent queries
• create indexes for table with more than one thousands records
• an accelerate SELECT, and make slower UPDATE, DELETE, INSERT
• delete useless indexes

Index type:

• b-tree
• hash (memory engine)