sql-server - View 上最大(日期)查询的奇怪查询计划

Question

我有一个包含 4 个年度表格的 View :

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE VIEW [dbo].[BGT_BETWAYDETAILS]
WITH SCHEMABINDING
AS
SELECT [bwd_BetTicketNr] ,
    [bwd_LineID] [int] ,
    [bwd_ResultID] [bigint] NOT NULL,
    [bwd_DateModified]  ,
    [bwd_DateModifiedTrunc]  ,
    [bwd_LineMaxPayout] 
FROM   [dbo].[BGT_BETWAYDETAILS_2020]
UNION ALL
SELECT [bwd_BetTicketNr] ,
    [bwd_LineID] [int] ,
    [bwd_DateModified]  ,
    [bwd_DateModifiedTrunc]  ,
    [bwd_LineMaxPayout] 
FROM   [dbo].[BGT_BETWAYDETAILS_2019]
UNION ALL
SELECT [bwd_BetTicketNr] ,
    [bwd_LineID] [int] ,
    [bwd_DateModified]  ,
    [bwd_DateModifiedTrunc]  ,
    [bwd_LineMaxPayout] 
FROM   [dbo].[BGT_BETWAYDETAILS_2018]
UNION ALL
SELECT [bwd_BetTicketNr] ,
    [bwd_LineID] [int] ,
    [bwd_DateModified]  ,
    [bwd_DateModifiedTrunc]  ,
    [bwd_LineMaxPayout] 
FROM   [dbo].[BGT_BETWAYDETAILS_2017];
GO

每个表都有以下结构:

CREATE TABLE [dbo].[BGT_BETWAYDETAILS_2020]
(
    [bwd_BetTicketNr] [bigint] NOT NULL,
    [bwd_LineID] [int] NOT NULL,
    [bwd_ResultID] [bigint] NOT NULL,
    [bwd_DateModified] [datetime] NULL,
    [bwd_DateModifiedTrunc] [date] NULL,
    [bwd_LineMaxPayout] [decimal](18, 4) NULL,

    CONSTRAINT [CSTR__BGT_BETWAYDETAILS_2020_CKEY] 
        PRIMARY KEY CLUSTERED ([bwd_BetTicketNr] ASC, [bwd_LineID] ASC, [bwd_ResultID] ASC)
                    WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, 
                          IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, 
                          ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

我已添加非聚集索引

CREATE NONCLUSTERED INDEX [NCI__DATEMODIFIED] 
ON [dbo].[BGT_BETWAYDETAILS_2020] ([bwd_DateModifiedTrunc] ASC)
                    WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, 
                          SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, 
                          ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

我正在运行以下 3 个查询:

SELECT COALESCE(MAX([bwd_DateModifiedTrunc]), '2019-01-01') AS next_date
FROM [dbo].[BGT_BETWAYDETAILS_2020]

SELECT COALESCE(MAX([bwd_DateModifiedTrunc]), '2019-01-01') AS next_date
FROM [dbo].[BGT_BETWAYDETAILS]

SELECT COALESCE(CAST(MAX([bwd_DateModified]) AS date), '2019-01-01') AS next_date
FROM [dbo].[BGT_BETWAYDETAILS]

第一个在每个年度表上运行时会立即运行。

第二个，似乎要花很长时间。这个查询计划看起来很奇怪。

该计划在每个年度表上显示两次索引扫描。

每个年度表的计划都是我期望看到的:

最后，非索引日期列上的计划也是我期望看到的(聚集索引扫描)。每个表上的聚集索引扫描。此查询运行时间约为 3 分钟，符合预期。

这里有什么问题吗？我缺少一些反模式吗？为什么非聚集索引上的索引扫描按照实时计划进行了2次？我希望 View 的响应速度与各个表的响应速度一样快。

郑重声明，我在 SQL Server 2017 上运行它。

Answer 1

这看起来像是一个优化器限制。我已提交 a suggestion that this should be improved .

一个更简单的例子是

CREATE TABLE T1(X INT NULL UNIQUE CLUSTERED);
CREATE TABLE T2(X INT NULL UNIQUE CLUSTERED);

INSERT INTO T1 
OUTPUT INSERTED.X INTO T2
SELECT TOP 100000 NULLIF(ROW_NUMBER() OVER (ORDER BY 1/0),1) 
FROM sys.all_objects o1, 
     sys.all_objects o2;

然后

WITH CTE AS
(
SELECT X FROM T1
UNION ALL
SELECT X FROM T2
)
SELECT MAX(X)
FROM CTE
OPTION (QUERYRULEOFF ScalarGbAggToTop)

这会禁用查询优化器规则ScalarGbAggToTop查询计划执行 MAX然后在每个单独的表上计算 MAX MAX的-es - 与

相同

SELECT MAX(MaxX)
FROM 
(
SELECT MAX(X) AS MaxX FROM T1 
UNION ALL
SELECT MAX(X) AS MaxX FROM T1 
) T

与 ScalarGbAggToTop启用规则的计划现在看起来像这样

它正在有效地执行以下操作...

SELECT MAX(MaxX)
FROM   (SELECT MAX(X) AS MaxX
        FROM   (SELECT TOP 1 X
                FROM   T1
                WHERE  X IS NULL
                UNION ALL
                SELECT TOP 1 X
                FROM   T1
                WHERE  X IS NOT NULL
                ORDER  BY X DESC) T1
        UNION ALL
        SELECT MAX(X) AS MaxX
        FROM   (SELECT TOP 1 X
                FROM   T2
                WHERE  X IS NULL
                UNION ALL
                SELECT TOP 1 X
                FROM   T2
                WHERE  X IS NOT NULL
                ORDER  BY X DESC) T2) T0 

...但是效率非常低。运行上面的 SQL 将给出一个带有查找的计划，并且每个分支仅读取一行。

ScalarGbAggToTop制作的计划仅对流聚合计划进行了最小的更改。看起来它从中进行扫描并对它应用向后排序，然后对 NOT NULL 使用向后排序。和NULL分支机构。并且不执行任何额外的探索来查看是否存在更高效的访问路径。

这意味着在病理情况下，所有行都是 NULL或NOT NULL其中一项扫描最终将读取表中的所有行(如果适用于所有 4 个表，则在您的情况下为 50 亿行)。即使有 NULL 的混合和NOT NULL事实上IS NULL分支正在进行向后扫描不是最优的，因为 NULL在 SQL Server 中排在第一位，因此位于索引的开头。

添加 NOT NULL首先分支似乎基本上是不必要的，因为如果没有它，查询将返回相同的结果。我想它只是需要它知道是否显示消息

Warning: Null value is eliminated by an aggregate or other SET operation.

但我怀疑你关心这个。在这种情况下添加显式 WHERE ... NOT NULL解决了问题。

WITH CTE AS
(
SELECT X FROM T1
UNION ALL
SELECT X FROM T2
)
SELECT MAX(X)
FROM CTE
WHERE X IS NOT NULL
;

它现在正在寻找 NOT NULL索引的一部分并向后读取(从每个表读取第一行后停止)