Question

我有查询，工作正常且速度很快（约1秒执行时间）：

array.emplace_back();

但是一旦我将第二行更改为：

SELECT COUNT(ticket) AS times_appears
    ,COUNT(LOGIN) AS number_of_accounts
    ,comment
FROM mt4_trades
WHERE COMMENT != ''
    AND CLOSE_TIME != '1970-01-01 00:00:00.000'
GROUP BY comment
ORDER BY times_appears DESC

查询正在减慢 20X 次。 ,COUNT(DISTINCT LOGIN) AS number_of_accounts这么慢是否会影响整个查询，或者我在这里遗漏了什么？

Answer 1

经过一些研究后我发现使用子查询有时比COUNT(DISTINCT column)更好。所以这是我的查询，比我问题上的查询快20倍：

SELECT COUNT(mtt.ticket) as times_appears
       --,COUNT(DISTINCT login) as number_of_accounts
       ,(SELECT COUNT(LOGIN) FROM (SELECT DISTINCT login FROM mt4_trades WHERE COMMENT=mtt.COMMENT AND CLOSE_TIME != '1970-01-01 00:00:00.000' ) AS temp)AS number_of_accounts
       ,comment
FROM mt4_trades mtt
WHERE mtt.COMMENT != ''
AND mtt.CLOSE_TIME != '1970-01-01 00:00:00.000'
GROUP BY mtt.comment
ORDER BY times_appears DESC

@Raphaël-Althau 感谢您提供有用的网址提示

Answer 2

---- tickt count, irrespective of login
Select   mtt.comment
        ,t.number_of_accounts
        ,Count(mtt.ticket) As times_appears
From    mt4_trades As mtt With (Nolock)
        Join
        (
            Select   t.comment
                    ,Count(t.login) As number_of_accounts
            From    (
                        Select  Distinct
                                 mtt.login
                                ,mtt.comment
                        From    mt4_trades As mtt With (Nolock)
                        Where   mtt.comment <> ''
                                And mtt.CLOSE_TIME <> '1970-01-01 00:00:00.000'
                    ) As t
            Group By t.comment
        ) As mt On mtt.comment = t.comment
Where   mtt.comment <> ''
        And mtt.CLOSE_TIME <> '1970-01-01 00:00:00.000'
Group By mtt.comment
        ,t.number_of_accounts

---- tickt count with respect to login
Select   t.comment
        ,Count(t.ticket) As times_appears
        ,Count(t.login) As number_of_accounts
From    (
            Select  Distinct
                     mtt.ticket
                    ,mtt.login
                    ,mtt.comment
            From    mt4_trades As mtt With (Nolock)
            Where   mtt.comment <> ''
                    And mtt.CLOSE_TIME <> '1970-01-01 00:00:00.000'
        ) As t
Group By t.comment

COUNT DISTINCT（列）使查询速度减慢20倍

2 个答案: