Question

我不明白为什么一个字段上的相同查询使用索引，而另一个字段上却不使用索引。这两个字段都有索引。一个字段是INT，另一个字段是TIMESTAMP。某种魔术。救命！

这是表格：

    CREATE TABLE `fb_posts` (
    `postID` BIGINT(20) NOT NULL AUTO_INCREMENT,
    `url` VARCHAR(255) NOT NULL,
    `time` INT(11) NOT NULL,
    `partnerID` INT(11) NOT NULL,
    `aTime` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    PRIMARY KEY (`postID`),
    INDEX `time` (`time`),
    INDEX `aTime` (`aTime`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
ROW_FORMAT=COMPRESSED
AUTO_INCREMENT=67640377;

说明查询：

1。

mysql>       explain SELECT
    ->                 `fb_posts`.`partnerID`
    ->             FROM
    ->                 `fb_posts`
    ->             WHERE
    ->                 `fb_posts`.`time` > 1551107400
    ->                 AND `fb_posts`.`time` <= 1551108000
    ->                 
    ->         ;
+----+-------------+----------+-------+---------------+------+---------+------+------+-----------------------+
| id | select_type | table    | type  | possible_keys | key  | key_len | ref  | rows | Extra                 |
+----+-------------+----------+-------+---------------+------+---------+------+------+-----------------------+
|  1 | SIMPLE      | fb_posts | range | time          | time | 4       | NULL |    1 | Using index condition |
+----+-------------+----------+-------+---------------+------+---------+------+------+-----------------------+
1 row in set (0.00 sec)

2。

mysql>       explain SELECT
    ->                 `fb_posts`.`partnerID`
    ->             FROM
    ->                 `fb_posts`
    ->             WHERE
    ->                 `fb_posts`.`aTime` > 1551107400
    ->                 AND `fb_posts`.`aTime` <= 1551108000
    -> ;
+----+-------------+----------+------+---------------+------+---------+------+----------+-------------+
| id | select_type | table    | type | possible_keys | key  | key_len | ref  | rows     | Extra       |
+----+-------------+----------+------+---------------+------+---------+------+----------+-------------+
|  1 | SIMPLE      | fb_posts | ALL  | aTime         | NULL | NULL    | NULL | 69567867 | Using where |
+----+-------------+----------+------+---------------+------+---------+------+----------+-------------+
1 row in set, 4 warnings (0.00 sec)

为什么？

Answer 1

优化器创建方式。他们使用统计信息来找出最适合此查询。

尝试查看统计信息并阅读手册“ PROCEDURE ANALYSE（）”

SELECT * FROM `fb_posts` PROCEDURE ANALYSE();

拒绝在TIMESTAMP字段上使用索引

1 个答案: