MySQL 不使用 ORDER BY 的索引

Question

我有一个名为“test”的简单 MySQL 表，有两列:

1. 名为“id”的自动递增 int 列
2. 名为“textcol”的 Varchar(3000) 列

我根据“textcol”列在表中创建索引。但是，ORDER BY 查询似乎没有使用索引，即在 textcol 上使用 ORDER BY 的简单查询上的 EXPLAIN 语句在其输出的 Key 列中显示 NULL，并且还使用文件排序。

任何用于进行更改以帮助使用 ORDER by 查询的索引的指针都会对我有用。

“mysql --version”命令给出的MySQL版本:

mysql Ver 14.14 Distrib 5.1.58，适用于使用 readline 6.2 的 debian-linux-gnu (x86_64)

mysql> CREATE TABLE test (id INTEGER NOT NULL AUTO_INCREMENT, PRIMARY KEY(id), textcol VARCHAR(3000));
Query OK, 0 rows affected (0.05 sec)

mysql> DESCRIBE test;
+---------+---------------+------+-----+---------+----------------+
| Field   | Type          | Null | Key | Default | Extra          |
+---------+---------------+------+-----+---------+----------------+
| id      | int(11)       | NO   | PRI | NULL    | auto_increment |
| textcol | varchar(3000) | YES  |     | NULL    |                |
+---------+---------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> CREATE INDEX textcolindex ON test (textcol);
Query OK, 0 rows affected, 2 warnings (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW INDEX FROM test;
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name     | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| test  |          0 | PRIMARY      |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |
| test  |          1 | textcolindex |            1 | textcol     | A         |        NULL |     1000 | NULL   | YES  | BTREE      |         |
+-------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
2 rows in set (0.00 sec)

mysql> INSERT INTO test (textcol) VALUES ('test1');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO test (textcol) VALUES ('test2');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO test (textcol) VALUES ('test3');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO test (textcol) VALUES ('test4');
Query OK, 1 row affected (0.00 sec)


mysql> EXPLAIN SELECT * FROM test ORDER BY textcol;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | test  | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT * FROM test ORDER BY id;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | test  | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

Answer 1

因为它必须加载整个表来回答查询并且对 4 个元素进行排序很便宜，查询优化器可能只是避免触及索引。更大的 table 还会发生这种情况吗？

请注意，varchar(3000) 列不能是覆盖索引，因为 MySQL 不会在索引中包含超过前 768 个左右字节的 varchar。

如果您希望查询只读取索引，则索引必须包含您要SELECT的每一列。在 innodb 上，一旦你使 textcol 足够小，它应该开始为你的两列表工作；在 MyISAM 上，您需要自己包含主键列，例如 CREATE INDEX textcolindex ON test (textcol,id);