Why is the leftmost subset of a key not being used to optimise this ORDER BY?

问题

Server version:

[root@cat best]# /usr/libexec/mysqld --version
/usr/libexec/mysqld  Ver 5.1.47 for redhat-linux-gnu on i386 (Source distribution)

Schema:

CREATE TABLE `Log` (
    `EntryId` INT UNSIGNED NOT NULL AUTO_INCREMENT,
    `EntryTime` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP(),
    `Severity` ENUM(
        'LOG_LEVEL_CRITICAL',
        'LOG_LEVEL_ERROR',
        'LOG_LEVEL_WARNING',
        'LOG_LEVEL_NOTICE',
        'LOG_LEVEL_INFO',
        'LOG_LEVEL_DEBUG'
    ) NOT NULL,

    `User` TEXT,
    `Text` TEXT NOT NULL,

    PRIMARY KEY(`EntryId`),
    KEY `TimeId` (`EntryTime`,`EntryId`)
) ENGINE=InnoDB COMMENT="Log of server activity";

Query:

SELECT 
   `EntryId`,
   `EntryTime`, -- or, ideally: UNIX_TIMESTAMP(`EntryTime`) AS `EntryTime_UnixTS`
   `Severity`,
   `User`,
   `Text`
FROM `Log` 
ORDER BY `EntryTime` DESC, `EntryId` DESC
LIMIT 0, 20

According to the execution plan (and observation), the index is not being used:

id  select_type  table  type  possible_keys  key  key_len  ref  rows  Extra
1   SIMPLE       Log    ALL   \N             \N   \N       \N   720   Using filesort

I've tried re-organising it a few ways with little success but, more than anything, would like to understand why this simple approach is failing. My understanding was that a left-most subset of any key can be used to optimise an ORDER BY operation.

Is my index wrong? Can I optimise the query?

Please note that I will also want to conditionally add, e.g.

WHERE `Severity` <= 'LOG_LEVEL_WARNING'

though I'd like to get the basic version working first if this makes the solution very different.

Reproduced on SQLFiddle under MySQL 5.5.32.

回答1:

The reason is that you index includes the primary key in it. and since it is InnoDB, by default the PK is ancluded in all other indexes as the left-most field. i.e. the index in this case is (EntryId, EntryTime, EntryId).

The solution is to have this index only on (EntryTime):

alter table Log drop index TimeId;
alter table Log add index TimeId(EntryTime);
explain SELECT     `EntryId`,    `EntryTime`,     `Severity`,    `User`,    `Text` FROM `Log`  ORDER BY `EntryTime` DESC, `EntryId` DESC LIMIT 0, 20;
+----+-------------+-------+-------+---------------+--------+---------+------+------+-------+
| id | select_type | table | type  | possible_keys | key    | key_len | ref  | rows | Extra |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-------+
|  1 | SIMPLE      | Log   | index | NULL          | TimeId | 4       | NULL |   20 | NULL  |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-------+

HTH

来源：https://stackoverflow.com/questions/26875660/why-is-the-leftmost-subset-of-a-key-not-being-used-to-optimise-this-order-by

标签

mysql

query-optimization

InnoDB