What table type is this? If it's MyISAM select count(*) should be instantaneous. If
it's InnoDb and you've got a large dataset (say 1 million+) then select count(*) will
be slow b/c it doesn't store the # of records in the table, thus it has to count
records. MyISAM does store the count so it
Something else to consider here:
MySQL can use indexes to optimize order by as well, but your order by value can't be
indexed since
it is derived from a database lookup/calculation. If you really need to order by
"diferenca"
you may have to live with it, although you may still see "Using fileso
