A MySQL EXPLAIN number of rows discrepancy

MySQL 5.5.49-log More questions on the query in https://dba.stackexchange.com/q/175377/5061 (the query is the same but the question is different): I have the following table (filled with many rows): CREATE TABLE SectorGraphs2 ( Kind tinyint(3) UNSIGNED NOT NULL COMMENT '1 - продюсер, 2 - жанр, 3 - регион', Criterion tinyint(3) UNSIGNED NOT NULL, Period tinyint(3) UNSIGNED NOT NULL, PeriodStart date NOT NULL, SectorID int(10) UNSIGNED NOT NULL, Value float NOT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin; ALTER TABLE SectorGraphs2 ADD UNIQUE KEY Producer2 (Kind,Criterion,Period,PeriodStart,SectorID) USING BTREE, ADD KEY SectorID (SectorID); then I run: EXPLAIN SELECT SectorID, SUM(Value) FROM SectorGraphs2 WHERE Kind = 1 AND Criterion = 7 AND Period = 1 AND PeriodStart >= ? AND PeriodStart < ? + INTERVAL 1 WEEK GROUP BY SectorID and it produces: +----+-------------+---------------+-------+---------------+-----------+---------+------+------+----------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra | +----+-------------+---------------+-------+---------------+-----------+---------+------+------+----------------------------------------------+ | 1 | SIMPLE | SectorGraphs2 | range | Producer2 | Producer2 | 6 | NULL | 1 | Using where; Using temporary; Using filesort | +----+-------------+---------------+-------+---------------+-----------+---------+------+------+----------------------------------------------+ See a nicely formatted explanation here . My question: Why it is used a temporary table and filesort but it reports only 1 row examined? It seems that because of using a temporary table, it should process more than one row. How can I determine the real number of rows processed? How to solve this discrepancy about number of processed rows? Note that the task I was assigned to do now is to eliminate heavy (involving too many rows) queries. And now I do not know how to do this.