Question

在多表全文布尔搜索中添加唯一键时，结果会循环显示3个任意状态中的1个，只有1个是正确的。

在检查下面的sqlfiddle时请记住这一点，因为查询可能最初正常工作 - 在这种情况下，在左侧面板中添加空格然后重建并重新运行 - 然后它应该被破坏（但它非常突然）。

以下是有问题的查询：

SELECT `i`.`item_id`, `g_a`.`alias` AS `group`, `i`.`name` AS `name`
  FROM `item` `i`
  JOIN `group_alias` `g_a` USING (group_id)
    WHERE
      MATCH (`g_a`.`alias`) AGAINST ('Mac*' IN BOOLEAN MODE)
    OR
      MATCH (`i`.`name`) AGAINST ('Mac*' IN BOOLEAN MODE);

足够简单。但是添加了以下唯一索引：

ALTER TABLE `item_with_unique` ADD UNIQUE INDEX `unique_item_group` (`group_id`, `name`)

结果在这三种状态之间任意循环：

返回所有行，就好像没有WHERE子句
返回别名匹配，就好像WHERE子句
返回正确的结果（根据我的经验，这是最罕见的）

行为似乎与这三种状态中的任何一种状态保持一致，直到查询以某种小的方式改变（比如添加括号）或者重构模式 - 此时它可能会发生变化。

这些限制是我在描述这种行为的MySQL文档中遗漏的吗？这是一个错误吗？或者我刚刚做了一些明显错误的事情？

Mysql版本5.6.35（撰写本文时为sqlfiddle）。

如果链接死亡，后代的Sqlfiddle：

CREATE TABLE `group` (
  `group_id` INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  `name` VARCHAR(256),
  FULLTEXT INDEX `search` (`name`)
) ENGINE = InnoDB;

CREATE TABLE `group_alias` (
  `group_id` INT UNSIGNED NOT NULL,
  `alias` VARCHAR(256),
  CONSTRAINT `alias_group_id`
    FOREIGN KEY (`group_id`)
    REFERENCES `group` (`group_id`),
  FULLTEXT INDEX `search` (`alias`)
) ENGINE = InnoDB;

CREATE TABLE `item` (
  `item_id` INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  `group_id` INT UNSIGNED,
  `name` VARCHAR(255) NOT NULL,
  CONSTRAINT `item_group_id`
    FOREIGN KEY (`group_id`)
    REFERENCES `group` (`group_id`),
  FULLTEXT INDEX `search` (`name`)
) ENGINE = InnoDB;

CREATE TABLE `item_with_unique` LIKE `item`;
ALTER TABLE `item_with_unique` ADD UNIQUE INDEX `unique_item_group` (`group_id`, `name`);

INSERT INTO `group` (`group_id`, `name`) VALUES (1, 'Thompson');
INSERT INTO `group` (`group_id`, `name`) VALUES (2, 'MacDonald');
INSERT INTO `group` (`group_id`, `name`) VALUES (3, 'Stewart');

INSERT INTO `group_alias` (`group_id`, `alias`) VALUES (1, 'Tomson');
INSERT INTO `group_alias` (`group_id`, `alias`) VALUES (2, 'Something');
INSERT INTO `group_alias` (`group_id`, `alias`) VALUES (3, 'MacStewart');

INSERT INTO `item` (`item_id`, `group_id`, `name`) VALUES (1, 1, 'MacTavish');
INSERT INTO `item` (`item_id`, `group_id`, `name`) VALUES (2, 1, 'MacTavish; Red');
INSERT INTO `item` (`item_id`, `group_id`, `name`) VALUES (3, 2, 'MacAgnew');
INSERT INTO `item` (`item_id`, `group_id`, `name`) VALUES (4, 3, 'Spider');
INSERT INTO `item` (`item_id`, `group_id`, `name`) VALUES (5, 2, 'blahblah');

INSERT INTO `item_with_unique` SELECT * FROM `item`;


SELECT `i`.`item_id`, `g_a`.`alias` AS `group`, `i`.`name` AS `name`,
IF(MATCH (`g_a`.`alias`) AGAINST ('Mac*' IN BOOLEAN MODE), 1, 0) AS `group_match`,
IF(MATCH (`i`.`name`) AGAINST ('Mac*' IN BOOLEAN MODE), 1, 0) AS `item_match`
  FROM `item` `i`
  JOIN `group_alias` `g_a` USING (group_id)
    WHERE
      MATCH (`g_a`.`alias`) AGAINST ('Mac*' IN BOOLEAN MODE)
    OR
      MATCH (`i`.`name`) AGAINST ('Mac*' IN BOOLEAN MODE);

SELECT "Same query, using table with unique index (NOTE: sporadically this is actually correct, in such case, skip to bottom notes)";
SELECT `i`.`item_id`, `g_a`.`alias` AS `group`, `i`.`name` AS `name`,
IF(MATCH (`g_a`.`alias`) AGAINST ('Mac*' IN BOOLEAN MODE), 1, 0) AS `group_match`,
IF(MATCH (`i`.`name`) AGAINST ('Mac*' IN BOOLEAN MODE), 1, 0) AS `item_match`
  FROM `item_with_unique` `i`
  JOIN `group_alias` `g_a` USING (group_id)
    WHERE
      MATCH (`g_a`.`alias`) AGAINST ('Mac*' IN BOOLEAN MODE)
    OR
      MATCH (`i`.`name`) AGAINST ('Mac*' IN BOOLEAN MODE);

SELECT "Union of the two OR match conditions seperately (expected result from second query)";
SELECT `i`.`item_id`, `g_a`.`alias` AS `group`, `i`.`name` AS `name`,
IF(MATCH (`g_a`.`alias`) AGAINST ('Mac*' IN BOOLEAN MODE), 1, 0) AS `group_match`,
IF(MATCH (`i`.`name`) AGAINST ('Mac*' IN BOOLEAN MODE), 1, 0) AS `item_match`
  FROM `item_with_unique` `i`
  JOIN `group_alias` `g_a` USING (group_id)
    WHERE
      MATCH (`g_a`.`alias`) AGAINST ('Mac*' IN BOOLEAN MODE)
UNION
SELECT `i`.`item_id`, `g_a`.`alias` AS `group`, `i`.`name` AS `name`,
IF(MATCH (`g_a`.`alias`) AGAINST ('Mac*' IN BOOLEAN MODE), 1, 0) AS `group_match`,
IF(MATCH (`i`.`name`) AGAINST ('Mac*' IN BOOLEAN MODE), 1, 0) AS `item_match`
  FROM `item_with_unique` `i`
  JOIN `group_alias` `g_a` USING (group_id)
    WHERE
      MATCH (`i`.`name`) AGAINST ('Mac*' IN BOOLEAN MODE);

SELECT "Now rebuild the schema (add a newline somewhere so sqlfiddle thinks it has changed) and observe that the results of the second query.  It may take multiple attempts but it usually cycles between 3 states:";
SELECT "1: Returns ALL results as if there were no conditions (5 rows)";
SELECT "2: Returns results as if there were no second part to the OR condition (1 row)";
SELECT "3: Returns the correct results (rarely)";

Answer 1

如果您有单字名称和别名。而且您正在检查整个值或主要值。那么FULLTEXT不是你需要的索引类型。

简单的INDEX(name)和name LIKE 'Mac%'将非常有效。

如果您有一个包含大量单词的长短语，并且“MacDonald”可能位于其中间，则然后 FULLTEXT和MATCH ... AGAINST是正确的方式去。

使用任何一种索引，

WHERE table1 ...
   OR table2 ...

效率低下。粗略地说，优化器必须进行“交叉连接”以获得两个表之间的所有行组合，然后查看哪些匹配一个或其他匹配/喜欢。

也许您对数据“过度规范化”了？ name和alias不能同时位于同一个表格中吗？查询将更快，和将有优化技术使其更快。只有1K行，你所拥有的东西会明显变慢;我提出的建议可以超过数百万甚至数十亿行进行优化。

Answer 2

尝试在您的声明中使用IGNORE INDEX：

SELECT `i`.`item_id`, `g_a`.`alias` AS `group`, `i`.`name` AS `name`
  FROM `item` `i`
  IGNORE INDEX (unique_item_group)
  JOIN `group_alias` `g_a` USING (group_id)
    WHERE
      MATCH (`g_a`.`alias`) AGAINST ('Mac*' IN BOOLEAN MODE)
    OR
      MATCH (`i`.`name`) AGAINST ('Mac*' IN BOOLEAN MODE);

MySQL非常笨拙，无法随机使用unique_item_group进行全文搜索。

MySQL零星的MATCH AGAINST具有唯一索引的行为

2 个答案: