- Indexes slow down inserts and updates, so you should use them carefully on columns that are FREQUENTLY updated.
- Indexes speed up where clauses and order by.
Remember to think about HOW your data is going to be used when building your tables.
There are a few other things to remember. If your table is very small, it's worse to use an index than to leave it out and just let it do a table scan. Indexes really only come in handy with tables that have a lot of rows.