Greetings
Having a table with the following structure -
+---------+---------------+-------------------+------+-----+---------+----------
------+
| Field | Type | Collation | Null |
Key | Default | Extra
|
+---------+---------------+-------------------+------+-----+---------+----------
------+
| email | char(50) | latin1_swedish_ci | YES |
MUL | NULL |
|
| is_sent | enum('Y','N') | latin1_swedish_ci | YES |
| NULL |
|
| id | int(10) | binary | |
PRI | NULL | auto_incr
ement |
+---------+---------------+-------------------+------+-----+---------+----------
------+
If I issue a command like this -
explain select substring(id,1,4) from email_table
where substring(id,1,4) < 300 limit 500;
The result returned is -
+----+-------------+-------------+-------+---------------+---------+---------+--
----+-------+--------------------------+
| id | select_type | table | type |
possible_keys | key | key_len | r
ef | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+--
----+-------+--------------------------+
| 1 | SIMPLE | email_table | index | NULL
| PRIMARY | 4 | N
ULL | 33914 | Using where; Using index |
+----+-------------+-------------+-------+---------------+---------+---------+--
----+-------+--------------------------+
This means it is using the index.
But, if issue a command like -
explain select email from email_table where
substring(id,1,4) < 300 limit 500;
+----+-------------+-------------+------+---------------+------+---------+------
+-------+-------------+
| id | select_type | table | type |
possible_keys | key | key_len | ref
| rows | Extra |
+----+-------------+-------------+------+---------------+------+---------+------
+-------+-------------+
| 1 | SIMPLE | email_table | ALL | NULL
| NULL | NULL | NULL
| 33914 | Using where |
+----+-------------+-------------+------+---------------+------+---------+------
+-------+-------------+
the KEY column is NULL i.e. it is not using Index?
Why the query is not using index?
Thanks in advance.
Karam
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