Historical version. PostgreSQL 9.4 has reached end of support. 2020-02-13. See the current manual.
57.5. Examples
The PostgreSQL source distribution includes several examples of index operator classes for SP-GiST. The core system currently provides radix trees over text columns and two types of trees over points: quad-tree and k-d tree. Look into src/backend/access/spgist/ to see the code.
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Read the upstream documentation. For corrections, first check the current manual.