create_users.sql - schema initialize schema gen.rb - generate sql for seed solution.sql - the solution

psql < create_users.sql
gen.rb | psql
psql < solution.sql

a bit faster solution in solution_lateral.sql

solution can be optimized further by:

• storing earth type coordinates directly as column, instead of lat, lon,
• using postgis, building convex hull for each group, then finding max distance in much smaller set of rows