Does PostGIS have separate implementation of GiST index from PostgreSQL?

I want to know if PostGIS has a separate implementation of GIST index or it reuses the GIST index implementation provided by Postgresql?

Answer

There's two things going on here: the GIST API in PostgreSQL and the bindings of types to that API for the purposes of building an R-Tree.

PostGIS necessarily uses the PostgreSQL GIST API. That's what it's for. That way we don't have to worry about transaction management or writing things to disk or all the other messy important things involved in maintaining your own index.

PostGIS has its own code to use the GIST API. There is code for building r-trees-on-gist in the PostgreSQL code base, but PostGIS does not use that code, we have our own, which is very similar (it's still aiming for an r-tree structure), but not identical.