Geo localized database RTREE
In order to get advantage of the SPATIAL index you have to filter results using a bounding box (or any other geometry type) with the functions st_within() or st_contains(). You have some examples here:


Query Optimization with MySQL 5.7 and MariaDB 10 - GIS Improvements
Using the new spatial functions in MySQL 5.6 for geo-enabled applications
MySQL 5.7 and GIS, an Example