Elizabeth Christensen has some neat imagery:
The secret to unlocking performance gains often lies not just in what you ask in a query, but in how Postgres finds the answer. The Postgres
EXPLAINsystem is great for understanding how data is being queried. One of secretes to reading EXPLAIN plans is understanding the type of scan done to retrieve the data. The scan type can be the difference between a lightning-fast response or a slow query.
Click through for the list, as well as images that clearly explain what’s happening.