EXPLAIN SELECT City.Name,Country.Name,City.Population FROM City, Country WHERE City.CountryCode = Country.Code AND City.Population > 8000000 ORDER BY City.Population; +---------+--------+---------------+---------+---------+------------------+------+-----------------------------+ | table | type | possible_keys | key | key_len | ref | rows | Extra | +---------+--------+---------------+---------+---------+------------------+------+-----------------------------+ | City | ALL | NULL | NULL | NULL | NULL | 4079 | Using where; Using filesort | | Country | eq_ref | PRIMARY | PRIMARY | 3 | City.CountryCode | 1 | | +---------+--------+---------------+---------+---------+------------------+------+-----------------------------+ %system%: table only has one row %const%: query matches at most one row from this table %eq_ref%: one row from this table will match from previous tables %ref%: rows matching from this table will match reference from earlier table %ref_or_null%: like ref, but also matching null references (used by subqueries) %range%: range of rows will be retrieved %index%: the entire index will be scanned %ALL%: the entire table will be scanned ALTER TABLE City ADD INDEX (population); +---------+--------+---------------+------------+---------+------------------+------+-------------+ | table | type | possible_keys | key | key_len | ref | rows | Extra | +---------+--------+---------------+------------+---------+------------------+------+-------------+ | City | range | Population | Population | 4 | NULL | 78 | Using where | | Country | eq_ref | PRIMARY | PRIMARY | 3 | City.CountryCode | 1 | | +---------+--------+---------------+------------+---------+------------------+------+-------------+