[Tile-serving] [openstreetmap/osm2pgsql] Use SP-GiST indexes when available (#1357)

[mboeringa]

@pnorman , yes, you can use SP-GiST quite successfully for indexing line type tables, but be aware of this limitation:

You cannot use an SP-GiST type spatial index on the 'way' column as input for the PostgreSQL CLUSTER command. It is quite coincidental, but I just found this out today attempting to use CLUSTER against a line table with an SP-GiST index. PostgreSQL returned a:

_"cannot cluster on index <INDEX_NAME>" because access method does not support clustering"_

type error. I already new this was the case with BRIN, and for BRIN this limitation is a logical consequence of the techical structure of the index, which excludes it as input type for a clustering operation, but hadn't expected the same limitation for SP-GiST.

So it seems PostgreSQL is currently only capable of using GiST type indexes as input for CLUSTER.

And you are probably also aware of the ongoing work for re-writing the GiST implementation in PostgreSQL, to make indexing (potentially parallel and) much faster, e.g.:
https://commitfest.postgresql.org/31/2824/

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