How can I index the following query to avoid the full table scan?
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 |
+----+-------------+------------------+------+---------------+------+---------+------+-------+-------------+
I have modified the query as bellow then also I can see row id2 (UNION) is going for full table scan.
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 | |
+----+--------------+------------------+------+---------------+--------------+---------+-------+-------+-------------+
Since you are comparing the indexed column with the constant value,try to avoid that.
Refer here: http://dev.mysql.com/doc/refman/5.0/en/where-optimizations.html
Also I suggest a non_clustered index on fld1,fld2 to make this query perform faster
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