如何索引以避免全表掃描？

如何索引以下查詢以避免全表掃描？如何索引以避免全表掃描？

explain SELECT fld1, fld2 FROM tablename WHERE IdReceived > 0; 

+----+-------------+------------------+------+---------------+------+---------+------+-------+-------------+ 
| id | select_type | table   | type | possible_keys | key | key_len | ref | rows | Extra  | 
+----+-------------+------------------+------+---------------+------+---------+------+-------+-------------+ 
| 1 | SIMPLE  | tablename  | ALL |IdReceived _idx| NULL | NULL | NULL | 99617 | Using where | 
+----+-------------+------------------+------+---------------+------+---------+------+-------+-------------+

我已經修改了查詢作爲波紋管，然後我也可以看到排ID2（UNION）會爲全表掃描。

explain SELECT fld1,fld2 FROM tablename WHERE IdReceived=1 UNION SELECT fld1,fld2 FROM tablename WHERE IdReceived>=1; 
+----+--------------+------------------+------+---------------+--------------+---------+-------+-------+-------------+ 
| id | select_type | table   | type | possible_keys | key   | key_len | ref | rows | Extra  | 
+----+--------------+------------------+------+---------------+--------------+---------+-------+-------+-------------+ 
| 1 | PRIMARY  | tablename | ref | IdReceived _idx | IdReceived _idx | 4  | const | 8865 |    | 
| 2 | UNION  | tablename | ALL | IdReceived _idx | NULL   | NULL | NULL | 99617 | Using where | 
| NULL | UNION RESULT | <union1,2>  | ALL | NULL   | NULL   | NULL | NULL | NULL |    | 
+----+--------------+------------------+------+---------------+--------------+---------+-------+-------+-------------+

來源

2012-09-27 user1638016

試圖強行統計更新。它*有*適當的索引。查詢計劃員必須確定最好不要使用它。（另外，IdRecieved列中的值的分佈情況如何？） – 2012-09-27 06:28:43