SP-GiST

SP-GiST is an abbreviation for space-partitioned GiST. SP-GiST supports partitioned search trees, which facilitate development of a wide range of different non-balanced data structures, such as quad-trees, k-d trees, and suffix trees (tries). The common feature of these structures is that they repeatedly divide the search space into partitions that need not be of equal size. Searches that are well matched to the partitioning rule can be very fast:
https://www.postgresql.org/docs/current/static/spgist-intro.html.

Some of the information of the SP-GIST is taken from Purdue University's SP-GiST Indexing Project (https://www.cs.purdue.edu/spgist/).