COUNT DISTINCT（列）减慢查询20X

Question

我有这个查询的正常工作和快（1秒左右的执行时间）：

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 

，但只要我改变第二行：

,COUNT(DISTINCT LOGIN) AS number_of_accounts 

查询放缓20X倍。 DISTINCT是如此缓慢，影响整个查询或我在这里丢失的东西？

Answer 1

经过一番研究，我发现有时使用子查询比COUNT(DISTINCT column)更好。 所以这是我查询这是一个比我的问题更快的20X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 

@拉斐尔 - Althau谢谢你的帮助的URL，暗示

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