Question

任何人都可以帮我优化此查询吗？这需要15秒。如果我将WHERE子句移动到连接本身，仍然会得到相同的结果。

SELECT
`users`.`employeeID` as `username`,
`users`.`firstName` as `fname`,
`users`.`lastName` as `lname`
FROM `enrollment`
INNER JOIN `users` ON `enrollment`.`employeeID` = `users`.`employeeID`
WHERE `enrollment`.`number` = [int]
AND
`enrollment`.`term` = [int]
AND
(
`enrollment`.`status` = 'E'
OR
`enrollment`.`status` = 'M'
)
ORDER BY    
`users`.`lastName` ASC,
`users`.`firstName` ASC;

只要这个：

SELECT
`users`.`employeeID` as `username`,
`users`.`firstName` as `fname`,
`users`.`lastName` as `lname`
FROM `enrollment`
INNER JOIN
`users` ON `enrollment`.`employeeID` = `users`.`employeeID`
AND
`enrollment`.`number` = [int]
AND
`enrollment`.`term` = [int]
AND
(
`enrollment`.`status` = 'E'
OR
`enrollment`.`status` = 'M'
)
ORDER BY    
`users`.`lastName` ASC,
`users`.`firstName` ASC;

这是EXPLAIN结果

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  users   ALL PRIMARY NULL    NULL    NULL    52925   Using filesort
1   SIMPLE  enrollment  ref employeeID,number_term  employeeID  9   ezlrn.users.employeeID  2   Using where

Answer 1

要详尽地回答，我们需要查看您获得的EXPLAIN SELECT...(your query)查询计划，以及已经存在的索引结构。

假设你还没有索引（除了PRIMARY KEY之外），那就是问题，让我们看看如何掀起一些东西（可能进一步改进）：

SELECT
    `users`.`employeeID` as `username`,
    `users`.`firstName` as `fname`,
    `users`.`lastName` as `lname`
FROM `enrollment`
    INNER JOIN `users` ON `enrollment`.`employeeID` = `users`.`employeeID`
WHERE `enrollment`.`number` = [int]
    AND
    `enrollment`.`term` = [int]
    AND
    (
        `enrollment`.`status` = 'E'
    OR
    `enrollment`.`status` = 'M'
    )
    ORDER BY    
        `users`.`lastName` ASC,
        `users`.`firstName` ASC;

JOIN和enrollment.employeeID之间有一个users.employeeID子句。

所以你需要的前几个索引就在那里：

-- CREATE INDEX enrollment_ndx ON enrollment(employeeID); -- wait on this one
CREATE INDEX users_ndx ON users(employeeID);

然后WHERE子句包含一些可以有用索引的条件。所以enrollment的最终索引是：

CREATE INDEX enrollment_ndx ON enrollment(employeeID, number, term, status);

尝试在注册和用户上创建索引，然后重试（发布查询计划）。

注意：您还可以在查询中使用语法enrollment.status IN ('M', 'E')来简化它。

Answer 2

您需要enrollment表上包含（{至少部分）number，term和status的索引。您还需要employeeID上的employees索引（如果它不是主键，它应该是主键）。

CREATE INDEX idx1 ON enrollment (number, term, status);
CREATE INDEX idx2 ON users (employeeID); // if needed

这将确保mysql首先使用索引在enrollment表中查找并再次加入users表。

需要帮助优化连接

2 个答案: