#this cache will load the information from the seq_region_table, if any, to allow mapping
#between internal and external seq_region_id
sub _cache_seq_region_mapping{
my$self=shift;
#for a given core database, will return the schema_build information
my$schema_build=$self->db->_get_schema_build();
#prepare the query to get relation for the current database being used
my$sql='SELECT s.internal_seq_region_id, s.external_seq_region_id from seq_region_mapping s, mapping_set ms where ms.mapping_set_id = s.mapping_set_id and ms.schema_build="'.$schema_build.'"';