为什么使用CASE，多个COUNT比一个SUM快？

我想知道以下两种方法中哪一种速度更快：

1）三COUNT：

 SELECT Approved = (SELECT COUNT(*) FROM dbo.Claims d
                  WHERE d.Status = 'Approved'),
        Valid    = (SELECT COUNT(*) FROM dbo.Claims d
                    WHERE d.Status = 'Valid'),
        Reject   = (SELECT COUNT(*) FROM dbo.Claims d
                    WHERE d.Status = 'Reject')

2）SUM与FROM-clause：

SELECT  Approved = SUM(CASE WHEN Status = 'Approved' THEN 1 ELSE 0 END),
        Valid    = SUM(CASE WHEN Status = 'Valid'    THEN 1 ELSE 0 END),
        Reject   = SUM(CASE WHEN Status = 'Reject'   THEN 1 ELSE 0 END)
FROM dbo.Claims c;

令我惊讶的是差异如此之大。具有三个子查询的第一个查询立即返回结果，而第二种SUM方法则需要18秒。

Claims是从包含约1800万行的表中选择的视图。该ClaimStatus表的FK列上有一个索引，其中包含状态名称。

无论我使用COUNT还是使用它，为什么会有如此大的不同SUM？

执行计划：

• COUNT
• SUM

共有12个状态。这三个状态占所有行的7％。

这是实际视图，我不确定是否相关：

CREATE VIEW [dbo].[Claims]
AS
SELECT 
   mu.Marketunitname AS MarketUnit, 
   c.Countryname     AS Country, 
   gsp.Gspname       AS GSP, 
   gsp.Wcmskeynumber AS GspNumber, 
   sl.Slname         AS SL, 
   sl.Wcmskeynumber  AS SlNumber, 
   m.Modelname       AS Model, 
   m.Salesname       AS [Model-Salesname], 
   s.Claimstatusname AS [Status], 
   d.Work_order      AS [Work Order], 
   d.Ssn_number      AS IMEI, 
   d.Ssn_out, 
   Remarks, 
   d.Claimnumber     AS [Claim-Number], 
   d.Rma_number      AS [RMA-Number], 
   dbo.ToShortDateString(d.Received_Date, 1) AS [Received Date], 
   Iddata, 
   Fisl, 
   Fimodel, 
   Ficlaimstatus 
FROM Tabdata AS d 
   INNER JOIN Locsl AS sl 
           ON d.Fisl = sl.Idsl 
   INNER JOIN Locgsp AS gsp 
           ON sl.Figsp = gsp.Idgsp 
   INNER JOIN Loccountry AS c 
           ON gsp.Ficountry = c.Idcountry 
   INNER JOIN Locmarketunit AS mu 
           ON c.Fimarketunit = mu.Idmarketunit 
   INNER JOIN Modmodel AS m 
           ON d.Fimodel = m.Idmodel 
   INNER JOIN Dimclaimstatus AS s 
           ON d.Ficlaimstatus = s.Idclaimstatus 
   INNER JOIN Tdefproducttype 
           ON d.Fiproducttype = Tdefproducttype.Idproducttype 
   LEFT OUTER JOIN Tdefservicelevel 
                ON d.Fimaxservicelevel = Tdefservicelevel.Idservicelevel 
   LEFT OUTER JOIN Tdefactioncode AS ac 
                ON d.Fimaxactioncode = ac.Idactioncode 

该COUNT(*)版本可以简单地针对您选择的每个状态在状态列中查找一次索引，而该SUM(...)版本则需要十二次查找索引（唯一状态类型的总数）。

显然，寻找索引的三倍将比寻找索引的12倍快。

第一个计划需要238MB的内存授权，而第二个计划需要650MB的内存授权。这可能是更大的内存授权不能立即填补，使得查询慢得多。

将第二个查询更改为：

SELECT  Approved = SUM(CASE WHEN Status = 'Approved' THEN 1 ELSE 0 END),
        Valid    = SUM(CASE WHEN Status = 'Valid'    THEN 1 ELSE 0 END),
        Reject   = SUM(CASE WHEN Status = 'Reject'   THEN 1 ELSE 0 END)
FROM dbo.Claims c
WHERE c.Status = 'Approved'
    OR c.Status = 'Valid'
    OR c.Status = 'Reject';

这将使查询优化器消除75％的索引查找，并且将导致所需的内存授予减少，I / O要求降低以及结果生成时间缩短。

该SUM(CASE WHEN ...)构造实质上防止查询优化器将Status谓词下推到计划的索引查找部分。