This is an interesting article about strategies to use when traditional indexes just won't do, but for the love of the index please use MySQL's (or postgres' or sqlite's) built in spatial index for this particular class of problems. It will does this sort of thing much, much more efficiently than 99% of in house solutions.

https://dev.mysql.com/doc/refman/8.0/en/spatial-types.html

http://postgis.refractions.net/

https://sqlite.org/rtree.html