thing.sql

-- Works perfectly!
SELECT COUNT(*),
  ST_Union(geom) as location_union,
  ST_SnapToGrid(ST_Transform(ST_SnapToGrid(ST_Transform(geom, _ST_BestSRID(geom)), 10,10),4326),0.0001, 0.0001) AS snap_pt
FROM locations
GROUP BY snap_pt
HAVING COUNT(*) > 1;

-- But what's a decent way to figure out which groups contain a given user?
-- I can use the following query to get the list of users in an example ST_Union from above:
select distinct(user_id)
from locations 
where ST_Contains('0104000020E610000003000000010100000074EFE192E38656C019E25817B735404001010000006284F068E38656C019E25817B735404001010000002D431CEBE28656C068B3EA73B5354040', geom);

-- If it was only valid SQL
SELECT COUNT(*),
  ST_Union(geom) as location_union,
  ST_SnapToGrid(ST_Transform(ST_SnapToGrid(ST_Transform(geom, _ST_BestSRID(geom)), 10,10),4326),0.0001, 0.0001) AS snap_pt
FROM locations
GROUP BY snap_pt
HAVING COUNT(*) > 1
  and '1' in (
    select user_id from locations where ST_Contains(location_union)
  )
;