spatial index
 While executing OPTIMIZE TABLE on MyISAM tables the server re-creates the
 index file(s) in order to sort them physically by the key. This cannot be 
 done for R-tree indexes as it makes no sense.
 The server was not checking the type of the index and was accessing an 
 R-tree index as if it was a B-tree.
 Fixed by preventing sorting the index file if it contains an R-tree index.