EXPLAIN select * from test LEFT JOIN genome_builds.grch37_flank ON chr=chrom AND (position BETWEEN start AND stop);


id      select_type     table   type    possible_keys   key     key_len ref     rows    Extra
1       SIMPLE  test    index   NULL    chr     15      NULL    2       Using index
1       SIMPLE  grch37_flank    ref     chrom,chrom_only,pos_only       chrom   4       tmp.test.chr    1702398