Question

我的PHP代码中有一个嵌套循环需要很长时间，并且想知道有办法加速这个：

$sql = "SELECT * FROM table_1";
$result = mysqli_query($sql);
while($row = mysqli_fetch_array($result)){

    $sql2 = "
    SELECT count(*) 
FROM user_modules  
WHERE 
begin_ymdhi >= ".$date_from." AND 
begin_ymdhi <= ".$date_to." AND 
(
    completed_ymdhi IS NULL OR  
    completed_ymdhi = '' 
) AND 
user_id = ".$row['user_id']." AND 
task_id = ".$row['task_id']." AND 
    module_discon = 'N' 
    ";
}

外部查询获得1000行，内部必须计算10,000行 - 运行时间大约为115秒！有没有办法改进这个方法，使用不同的技术或组合的SQL查询？

Answer 1

不要使用嵌套查询，将它们组合成一个带有连接的查询：

SELECT t1.*, COUNT(u.user_id) ct
FROM table_1 t1
LEFT JOIN user_modules AS u ON u.user_id = t1.user_id AND u.task_id = t1.task_id
    AND u.begin_ymdhi BETWEEN '$date_from' AND '$date_to'
    AND u.module_discon = 'N'
GROUP BY t1.user_id, t1.task_id

Answer 2

SELECT user_modules.user_id, user_modules.task_id, count(*) 
FROM user_modules LEFT JOIN table_1 USING (user_id, task_id) 
WHERE  
begin_ymdhi >= ".$date_from." AND 
begin_ymdhi <= ".$date_to." AND 
    module_discon = 'N' AND
(
    completed_ymdhi IS NULL OR  
    completed_ymdhi = '' 
) 
GROUP BY user_modules.user_id, user_modules.task_id

在整个EXPLAIN语句（即SELECT）之前附加EXPLAIN SELECT count(*)...，MySQL会给你一个关于select正在做什么的破坏。

确保begin_ymdhi字段已正确编入索引。 SHOW INDEX FROM table_2要看。

Answer 3

task_id是唯一的吗？如果是这样，那么最直接的就是：

 $sql2 = "
    SELECT count(task_id) AS TaskCount
FROM user_modules  
WHERE 
begin_ymdhi >= ".$date_from." AND 
begin_ymdhi <= ".$date_to." AND 
(
    completed_ymdhi IS NULL OR  
    completed_ymdhi = '' 
) AND module_discon = 'N'  
group by user_id
";

$result = mysqli_query($sql2);

加快SQL循环

3 个答案: