So far, only binary trees have been discussed. However, in many cases, B-trees are just not enough. Why is that? As we've already discussed in this chapter, B-trees are basically based on sorting. The <, <=, =, >=, and > operators can be handled using B-trees. The trouble is, not every data type can be sorted in a useful way. Just imagine a polygon. How would you sort these objects in a useful way? Sure, you can sort by the area covered, its length, and so on, but doing this won't allow you to actually find them using a geometric search.
The solution to this problem is to provide more than just one index type. Each index will serve a special purpose and do exactly what is needed. The following six index types are available (as of PostgreSQL 10.0):
test=# SELECT * FROM pg_am;
oid | amname | amhandler | amtype
------+--------+----------------------+--------
2 | heap | heap_tableam_handler | t
403 | btree | bthandler...
