为什么SQL Server会选择“非群集索引扫描”？

在我查询的其中一个表中，聚簇索引是通过不是主键的键创建的。（我不知道为什么。）为什么SQL Server会选择“非群集索引扫描”？

但是，此表的主键存在非聚集索引。

在执行计划中，SQL选择聚簇索引，而不是我在查询中使用的主键的非聚簇索引。

SQL是否会这样做？我如何强制SQL选择非聚集索引呢？

追加更多详细信息：

表有许多领域和查询包含许多联接。让我抽象一下。

表定义是这样的：

SlowTable 
[SlowTable_id] [int] IDENTITY(200000000,1) NOT NULL, 
[fk1Field] [int] NULL, 
[fk2Field] [int] NULL, 
[other1Field] [varchar] NULL, 
etc. etc...

，然后该表的指标是：

fk1Field (Clustered) 
SlowTable_id (Non-Unique, Non-Clustered) 
fk2Field (Non-Unique, Non-Clustered) 
... and 14 other Non-Unique, Non-Clustered indices on other fields

想必有很多反对fk1Field这就是为什么他们选择这个作为更加查询聚集指数的基础。

查询我使用一个观点：

SELECT 
    [field list] 
FROM 
    SourceTable1 S1 
    INNER JOIN SourceTable2 S2 
     ON S2.S2_id = S1.S2_id 
    INNER JOIN SourceTable3 S3 
     ON S3.S3_id = S2.S3_id 
    INNER JOIN SlowTable ST 
     ON ST.SlowTable_id = S1.SlowTable_id 
    INNER JOIN [many other tables, around 7 more...]

执行计划是相当大的，与有关的节点说

Hash Match 
(Inner Join) 
Cost: 9%

用粗箭头指向

Clustered Index Scan (Clustered) 
SlowTable.fk1Field 
Cost: 77%

我希望这能够提供足够的细节。

谢谢！

附录2：更正以前我的帖子。该视图没有where子句。这只是一系列内部连接。执行计划是从一个Insert语句中获取的，该语句在复杂查询中使用View（列为SLOW_VIEW），如下所示：

（此存储过程所做的是执行一些比例拆分记录的基础上，权重，计算针对总百分比这模仿，比如说，从一个帐户分配一个值，其他帐户）

INSERT INTO dbo.WDTD(
    FieldA, 
    FieldB, 
    GWB_id, 
    C_id, 
    FieldC, 
    PG_id, 
    FieldD, 
    FieldE, 
    O_id, 
    FieldF, 
    FieldG, 
    FieldH, 
    FieldI, 
    GWBIH_id, 
    T_id, 
    JO_id, 
    PC_id, 
    PP_id, 
    FieldJ, 
    FieldK, 
    FieldL, 
    FieldM, 
    FieldN, 
    FieldO, 
    FieldP, 
    FieldQ, 
    FieldS) 
SELECT DISTINCT 
    @FieldA FieldA, 
    GETDATE() FieldB, 
    @Parameter1 GWB_id, 
    GWBIH.C_id C_id, 
    P.FieldT FieldC, 
    P.PG_id PG_id, 
    PAM.FieldD FieldD, 
    PP.FieldU FieldE, 
    GWBIH.O_id O_id, 
    CO.FieldF FieldF, 
    CO.FieldG FieldG, 
    PSAM.FieldH FieldH, 
    PSAM.FieldI FieldI, 
    SOURCE.GWBIH_id GWBIH_id, 
    ' ' T_id, 
    GWBIH.JO_id JO_id, 
    SOURCE.PC_id PC_id, 
    GWB.PP_id, 
    SOURCE.FieldJ FieldJ, 
    1 FieldK, 
    ROUND((SUM(GWBIH.Total)/AGG.Total) * SOURCE.Total, 2) FieldL, 
    ROUND((SUM(GWBIH.Total)/AGG.Total) * SOURCE.Total, 2) FieldM, 
    0 FieldN, 
    ' ' FieldO, 
    ESGM.FieldP_flag FieldP, 
    SOURCE.FieldQ FieldQ, 
    '[UNPROCESSED]' 
FROM 
    dbo.Table1 GWBIH 
    INNER JOIN dbo.Table2 GWBPH 
     ON GWBPH.GWBP_id = GWBIH.GWBP_id 
    INNER JOIN dbo.Table3 GWB 
     ON GWB.GWB_id = GWBPH.GWB_id 
    INNER JOIN dbo.Table4 P 
     ON P.P_id = GWBPH.P_id 
    INNER JOIN dbo.Table5 ESGM 
     ON ESGM.ET_id = P.ET_id 
    INNER JOIN dbo.Table6 PAM 
     ON PAM.PG_id = P.PG_id 
    INNER JOIN dbo.Table7 O 
     ON O.dboffcode = GWBIH.O_id 
    INNER JOIN dbo.Table8 CO 
     ON 
      CO.Country_id = O.Country_id 
      AND CO.Brand_id = O.Brand_id 
    INNER JOIN dbo.Table9 PSAM 
     ON PSAM.Office_id = GWBIH.O_id 
    INNER JOIN dbo.Table10 PCM 
     ON PCM.PC_id = GWBIH.PC_id 
    INNER JOIN dbo.Table11 PC 
     ON PC.PC_id = GWBIH.PC_id 
    INNER JOIN dbo.Table12 PP 
     ON PP.PP_id = GWB.PP_id 
      -- THIS IS THE VIEW THAT CONTAINS THE CLUSTERED INDEX SCAN 
    INNER JOIN dbo.SLOW_VIEW GL 
     ON GL.JO_id = GWBIH.JO_id 
    INNER JOIN (
     SELECT 
      GWBIH.C_id C_id, 
      GWBPH.GWB_id, 
      SUM(GWBIH.Total) Total 
     FROM 
      dbo.Table1 GWBIH 
      INNER JOIN dbo.Table2 GWBPH 
       ON GWBPH.GWBP_id = GWBIH.GWBP_id 
      INNER JOIN dbo.Table10 PCM 
       ON PCM.PC_id = GWBIH.PC_id 
     WHERE 
      PCM.Split_flag = 0 
      AND GWBIH.JO_id IS NOT NULL 
     GROUP BY 
      GWBIH.C_id, 
      GWBPH.GWB_id 
      ) AGG 
     ON AGG.C_id = GWBIH.C_id 
      AND AGG.GWB_id = GWBPH.GWB_id 
    INNER JOIN (
     SELECT 
      GWBIH.GWBIH_id GWBIH_id, 
      GWBIH.C_id C_id, 
      GWBIH.FieldQ FieldQ, 
      GWBP.GWB_id GWB_id, 
      PCM.PC_id PC_id, 
      CASE 
      WHEN WT.FieldS IS NOT NULL 
       THEN WT.FieldS 
      WHEN WT.FieldS IS NULL 
       THEN PCMS.FieldT 
      END FieldJ, 
      SUM(GWBIH.Total) Total 
     FROM 
      dbo.Table1 GWBIH 
      INNER JOIN dbo.Table2 GWBP 
       ON GWBP.GWBP_id = GWBIH.GWBP_id 
      INNER JOIN dbo.Table4 P 
       ON P.P_id = GWBP.P_id 
      INNER JOIN dbo.Table10 PCM 
       ON PCM.PC_id = GWBIH.PC_id 
      INNER JOIN dbo.Table11 PCMS 
       ON PCMS.PC_id = PCM.PC_id 
      LEFT JOIN dbo.WT WT 
       ON WT.ET_id = P.ET_id 
       AND WT.PC_id = GWBIH.PC_id 
     WHERE 
      PCM.Split_flag = 1 
     GROUP BY 
      GWBIH.GWBI_id, 
      GWBIH.C_id, 
      GWBIH.FieldQ, 
      GWBP.GWB_id, 
      WT.FieldS, 
      PCM.PC_id, 
      PCMS.ImportCode 
      ) SOURCE 
     ON SOURCE.C_id = GWBIH.C_id 
      AND SOURCE.GWB_id = GWBPH.GWB_id 
WHERE 
    PCM.Split_flag = 0 
    AND AGG.Total > 0 
    AND GWBPH.GWB_id = @Parameter1 
    AND NOT EXISTS (
     SELECT * 
     FROM dbo.WDTD 
     WHERE 
      TD.C_id = GWBIH.C_id 
      AND TD.FieldA = GWBPH.GWB_id 
      AND TD.JO_id = GWBIH.JO_id 
      AND TD.PC_id = SOURCE.PC_id 
      AND TD.GWBIH_id = ' ') 
GROUP BY 
    GWBIH.C_id, 
    P.FieldT, 
    GWBIH.JO_id, 
    GWBIH.O_id, 
    GWBPH.GWB_id, 
    P.PG_id, 
    PAM.FieldD, 
    PP.FieldU, 
    GWBIH.O_id, 
    CO.FieldF, 
    CO.FieldG, 
    PSAM.FieldH, 
    PSAM.FieldI, 
    GWBIH.JO_id, 
    SOURCE.PC_id, 
    GWB.PP_id, 
    SOURCE.FieldJ, 
    ESGM.FieldP_flag, 
    SOURCE.GWBIH_id, 
    SOURCE.FieldQ, 
    AGG.Total, 
    SOURCE.Total

附录3：在做的一个执行计划选择声明的观点，我看到这个：

Hash Match  <==== Bitmap   <------ etc... 
(Inner Join)   (Bitmap Create) 
Cost: 0%    Cost: 0% 
    ^
    | 
    | 
Parallelism       Clustered Index Scan (Clustered) 
(Repartition Streams) <====  Slow_Table.fk1Field 
Cost: 1%        Cost: 98%

附录4：我想我发现了这个问题。聚集索引扫描不是指引用主键的my子句，而是另一个需要一个字段的子句，它在某种程度上与上面的fk1Field相关。

来源

2011-06-09 Jonas Arcangel

需要更多信息：表+索引+计划 – gbn 2011-06-09 07:47:24

有很多可能性，其范围不可能在没有进一步信息的情况下精确缩小，如相关的模式信息，实际执行计划，查询文本等。 – 2011-06-09 07:48:20

info *不够*你的WHERE和GROUP BY子句是什么？另外，每个索引都是单列索引吗？ – gbn 2011-06-09 10:03:26