IN clause not using index

Question

Here is the table definition

CREATE TABLE `dt_prdtime` (
  `TCompany` varchar(3) NOT NULL DEFAULT \'\',
  `TPerCode` varchar(8) NOT NULL,
  `TBegDateTime` dateti         


        

Answer 1

I will go out on a limb and say it is because you are using the MyISAM engine.

It is working perfectly fine with INNODB as can be seen in this Answer of mine.

I will try to spook up at least 1 honorable reference on the matter.

Here, The range Join Type, clearly an INNODB focus as it is the default engine. And when not explicitly mentioned in the manual in some documentation hierarchy, it is assumed.

Note, there is nothing contiguous about the id's in my example link. Meaning, don't hyperfocus on type=range in its EXPLAIN output. The speed is arrived at via the Optimizer (the CBO).

The cardinality in my example is very high (4.3 Million). The target id counts are relatively low (1000). The index is used.

Your situation may be the opposite: your cardinality might be incredibly low, like 3, and the optimizer decides to abandon use of the index.

To check your index cardinality, see the Manual Page SHOW INDEX Syntax.

A simple call such as:

show index from ratings;

+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table   | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| ratings |          0 | PRIMARY  |            1 | id          | A         |     4313544 |     NULL | NULL   |      | BTREE      |         |               |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+