Let's move on to an important issue: not everything can be sorted easily and in a useful way. Have you ever tried to sort circles? If the question seems odd, just try to do it. It will not be easy and will be highly controversial, so how do we do it best? Would we sort by size or coordinates? Under any circumstances, using a B-tree to store circles, points, or polygons might not be a good idea at all. A B-tree does not do what you want it to do because a B-tree depends on some kind of sorting order.

To provide end users with maximum flexibility and power, PostgreSQL provides more than just one index type. Each index type supports certain algorithms used for different purposes. The following list of index types is available in PostgreSQL (as of Version 9.4.1):