Note to self: Remember to add indices on those columns that will be used in queries as keys.
Links to index-related documentation in each of the popular databases:
- MySQL 5.5: How MySQL Uses Indexes
- PostgreSQL 9.1: Index Types
- MongoDB: Indexes
- SQLite 3: The SQLite Query Planner
As all the documentation linked above states, an index is something that can easily increase performance if designed properly.
Therefore, do only proper assigning of indices.