Question

有人可以告诉我如何使用加速这个mysql查询。我知道由于count（distinct subcontractorRef）语法，它目前运行缓慢。还有其他选择吗？

SELECT DISTINCT (contractorsRef) AS cref, RIDGROUP AS ridg,  
    (select count(DISTINCT subcontractorRef) 
    FROM request 
    INNER JOIN request_config ON request_config.RIDGROUP = request.RIDGROUP 
    WHERE request_config.contractorsRef = outer_config.contractorsRef  
    AND currenttaxyear =2011 
    AND weekno =31) AS xxx 
FROM request_config outer_config 
WHERE currenttaxyear =2011 
AND weekno =32 
AND contractorsRef <>132

Answer 1

使用JOIN而不是子查询来尝试：

SELECT
    contractorsRef AS cref,
    RIDGROUP AS ridg,
    IFNULL(T1.subcontractorCount, 0) AS xxx
FROM request_config AS outer_config
LEFT JOIN
(
    SELECT
        request_config.contractorsRef,
        COUNT(DISTINCT subcontractorRef) AS subcontractorCount
    FROM request 
    INNER JOIN request_config
    ON request_config.RIDGROUP = request.RIDGROUP 
    AND currenttaxyear = 2011 
    AND weekno = 31
    GROUP BY contractorsRef
) T1
ON T1.contractorsRef = outer_config.contractorsRef 
WHERE currenttaxyear = 2011 
AND weekno = 32 
AND contractorsRef <> 132
GROUP BY outer_config.contractorsRef

Answer 2

猜测，因为我们不知道任何索引。但是一个非常常见的错误是没有索引覆盖where子句中的表。外部选择的例子：

FROM request_config 在哪里currenttaxyear = 2011 AND weekno = 32 AND contractorsRef＆lt;＆gt; 132

然后你需要一个索引：在request_config上创建索引mynewIndex（currenttaxyear，weekno，contractorsRef）该索引中列的顺序可以产生很大的不同。因此，尝试围绕升技改变它们。

当使用count（distinct）查询运行select时，如何加快mysql查询速度？

2 个答案: