What exactly is SOLR?
Solr uses lucene to do the spatial and full text indexing, not mysql. It basically wraps a REST-style service on top of lucene's index library so you can access it as a service. The wikipedia article is pretty good for an overview: http://en.wikipedia.org/wiki/Apache_Solr

An alternative to Solr that also uses lucene is Elastic Search, which I think is superior in at least a few ways--see this answer from the creator of Elastic Search: https://stackoverflow.com/questions/2271600/elasticsearch-sphinx-lucene-solr-xapian-which-fits-for-which-usage/2288211#2288211