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我在两个必须加入的大表上创建了唯一的聚集索引，但是查询计划证明这些索引没有被使用。当我通过提示强制使用这些索引时，会使用索引扫描并且性能会变得更糟。一个表的唯一键是第二个表的外键，所以这让我很困惑。这是架构。这两个表是LOC和POL。LOC有 700 万行奇数行，POL超过 600 万行。

CREATE TABLE [dbo].[LOC](
    [acct_num] [char](30) NOT NULL,
    [cntr_num] [char](30) NOT NULL,
    [lob_cde] [char](2) NOT NULL,
    [ste_locn_nme] [char](30) NOT NULL,
    [buldg_num] [char](20) NOT NULL,
    [prctr_cde] [char](3) NULL,
        ...more fields...
) ON [PRIMARY]

CREATE NONCLUSTERED INDEX [IDX_All] ON [dbo].[LOC] 
(
    [acct_num] ASC,
    [cntr_num] ASC,
    [lob_cde] ASC,
    [buldg_num] ASC,
    [prctr_cde] ASC,
    [spcl_cond_1_id] ASC,
    [spcl_cond_2_id] ASC,
    [spcl_cond_3_id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

CREATE UNIQUE CLUSTERED INDEX [IDX_LOC_PKEY] ON [dbo].[LOC] 
(
    [acct_num] ASC,
    [lob_cde] ASC,
    [prctr_cde] ASC,
    [cntr_num] ASC,
    [ste_locn_nme] ASC,
    [buldg_num] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

... more non-unique indices on LOC, each on single columns: buldg_num, prctr_cde, acct_num

CREATE TABLE [dbo].[POL](
    [acct_num] [char](30) NOT NULL,
    [cntr_num] [char](30) NOT NULL,
    [lob_cde] [char](2) NOT NULL,
    [prctr_cde] [char](3) NULL,
        ...more fields...
) ON [PRIMARY]

CREATE NONCLUSTERED INDEX [IDX_All] ON [dbo].[POL] 
(
    [acct_num] ASC,
    [cntr_num] ASC,
    [lob_cde] ASC,
    [prctr_cde] ASC,
    [acct_nme] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

CREATE UNIQUE CLUSTERED INDEX [IDX_POL_PKEY] ON [dbo].[POL] 
(
    [acct_num] ASC,
    [lob_cde] ASC,
    [prctr_cde] ASC,
    [cntr_num] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

... more non-unique indices on POL, each on single columns: cntr_num, prctr_cde

如您所见，POL 表唯一键 ( acct_num, lob_cde, prctr_cde, cntr_num) 的四个字段是LOC表主键的前四列。这是我要运行的一个查询，就像许多将加入这两个表的查询一样：

select 
      [Easy matches] = COUNT(*)
FROM LOC INNER JOIN POL ON (
        LOC.acct_num = POL.acct_num 
    AND LOC.lob_cde = POL.lob_cde 
    AND LOC.prctr_cde = POL.prctr_cde
    AND LOC.cntr_num = POL.cntr_num)

没有提示，这喜欢使用IDX_prctr_cde每个表中的索引。该prctr_cde列的选择性不是很高；LOCorPOL表中只有七个不同的值。如果我暗示查询应该使用IDX_cntr_num索引，我会得到很好的性能，因为它是一个高度选择性的列（每个表中有超过 600 万个不同的值）。acct_num几乎与 一样有选择性cntr_num，也有超过 600 万个不同的值。

为什么默认使用非选择性索引？为什么切换到使用唯一的聚集索引会使查询运行得更慢？（慢 10 倍、20 倍甚至 30 倍。）

注意：我使用的提示是：

OPTION ( 
        TABLE HINT(POL, INDEX (IDX_POL_PKEY)),
        TABLE HINT(LOC, INDEX (IDX_LOC_PKEY))
       )

注意：我使用的是 SQL Server 2005 和 SQL Server 2008。

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