Adding indexes to a table is a very powerful way to improve the query performance. An index allows MySQL to quickly find the data needed for a query. When the right indexes are added to your tables, query performance can potentially be improved by several orders of magnitude. The trick is to know which indexes to add. Why not just add indexes on all columns? Indexes have overhead as well, so you need to analyze your needs before adding random indexes.
This chapter starts out discussing what an index is, some index concepts, and what drawbacks adding an index can have. Then ...
