Question

这是非常简单的3张桌子。我希望优化程序在contract.id上使用索引并拾取1行，并在拾取后加入其他表。但是优化器不使用索引。为什么？

如何优化此查询以使用索引？

explain SELECT c.*, o.name FROM contract c 
LEFT JOIN user u ON(c.user=u.id) 
LEFT JOIN office o ON(u.office=o.id) 
ORDER BY c.id LIMIT 0, 1

------------------ EXPLAIN结果-------------------

id  select_type table   partitions  type    possible_keys   key key_len     ref   rows  filtered    Extra
1   SIMPLE      c       NULL      ALL       NULL          NULL      NULL    NULL    12  100     Using temporary; Using filesort
1   SIMPLE      u       NULL      ALL       PRIMARY       NULL      NULL    NULL     5  100     Using where; Using join buffer (Block Nested Loop)
1   SIMPLE      o       NULL      eq_ref    PRIMARY       PRIMARY      4    u.office    1   100 NULL

------------------ MYSQL数据-------------------

CREATE TABLE `contract` (
  `id` int(11) NOT NULL,
  `user` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `contract` (`id`, `user`) VALUES
(1, 1),
(2, 3),
(3, 3),
(4, 2),
(5, 1),
(6, 3),
(7, 5),
(8, 3),
(9, 2),
(10, 1),
(11, 2),
(12, 3);


CREATE TABLE `office` (
  `id` int(11) NOT NULL,
  `name` varchar(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


INSERT INTO `office` (`id`, `name`) VALUES
(1, 'tokyo'),
(2, 'osaka'),
(3, 'kyoto'),
(4, 'fukushima'),
(5, 'hiroshima');


CREATE TABLE `user` (
  `id` int(11) NOT NULL,
  `office` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `user` (`id`, `office`) VALUES
(1, 3),
(2, 2),
(3, 1),
(4, 4),
(5, 3);

ALTER TABLE `contract`  ADD PRIMARY KEY (`id`) USING BTREE;
ALTER TABLE `office`    ADD PRIMARY KEY (`id`) USING BTREE;
ALTER TABLE `user`      ADD PRIMARY KEY (`id`) USING BTREE;
ALTER TABLE `contract`  MODIFY `id` int(11) NOT NULL AUTO_INCREMENT;
ALTER TABLE `office`    MODIFY `id` int(11) NOT NULL AUTO_INCREMENT;
ALTER TABLE `user`      MODIFY `id` int(11) NOT NULL AUTO_INCREMENT;

如何在多个表上优化mysql JOIN（不使用索引）

0 个答案: