gravitystorm left a comment (openstreetmap/openstreetmap-website#6512)
> Are there any past instances of features that haven't been implemented, or
> have been implemented with workarounds, because PostGIS wasn't available?
For historical reasons (e.g. timing of when we moved to Postgres, widespread
availability of PostGIS etc) the "nearby users" feature has been implemented
using our own code. See for example
[`User#nearby`](https://github.com/openstreetmap/openstreetmap-website/blob/3023ef046a102226c60237634d8e11c5d6ae9a5b/app/models/user.rb#L299-L314),
[`sql_for_distance`](https://github.com/openstreetmap/openstreetmap-website/blob/3023ef046a102226c60237634d8e11c5d6ae9a5b/lib/osm.rb#L465-L467),
[`sql_for_area`](https://github.com/openstreetmap/openstreetmap-website/blob/3023ef046a102226c60237634d8e11c5d6ae9a5b/lib/osm.rb#L521-L528)
and the [QuadTile](https://github.com/openstreetmap/quad_tile) library. It's a
prime candidate for refactoring to use PostGIS nowadays, but we should discuss
that elsewhere. As for features that haven't yet been implemented, I have lots
of ideas (e.g. "notes near me") that again would be candidates for implementing
with PostGIS, but again we should discuss elsewhere.
Returning to the matter at hand, I believe it can be implemented using PostGIS,
without adding any additional columns to the nodes/ways/relations or changesets
tables. I would attempt an implementation as follows:
* create a "zones" table (name tbd, I'm just using a distinctive one here to
help illustration) with usual attributes, created_by etc and a PostGIS geometry
column
* Have a form for creating zones, with something like https://terradraw.io/ to
create the geometry attribute
* Hook into the
[`Changeset#update_bbox`](https://github.com/openstreetmap/openstreetmap-website/blob/3023ef046a102226c60237634d8e11c5d6ae9a5b/app/models/changeset.rb#L145-L153)
method, and throw an error if the changeset overlaps any zone
* This can be detected by creating a postgis bbox for the changeset at
query-time, and relying on the PostGIS indexes and intersection tests to
efficiently check for overlapping zones, e.g. (pseudo-query) `select * from
zones where ST_Intersects(ST_MakeEnvelope(#{changeset.min_lon}, #{...}, ...),
zones.geometry)` or similar
As for whether we should use PostGIS or continue writing/using our own spatial
code, I think PostGIS is my preferred choice for this. It moves all the heavy
lifting (e.g. 2D-indexes, intersection testing) to a well-maintained library,
and if we only need ~1 line of SQL to detect the overlaps, it's much easier to
implement both here and in cgimap with much duplication.
There's a few details to be worked out (e.g. exact type of the geometry column,
should zones auto-expire like UserBlocks etc) but the spatial stuff is
important to nail down first.
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