RANK() SQL 服务器执行计划问题

Question

什么驱使 SQL 服务器对返回 6000 多行的查询使用不太理想的执行计划？我需要提高返回所有行的场景的查询性能。

我 select 所有字段并在索引中包含的相同三列上添加排名。根据返回的行数，查询有两个不同的执行计划，因此执行分别需要 0.2 秒或 3 秒。

从第 1 行返回到 ca。 5000 条查询运行速度很快。从 6000 行返回到全部，查询运行缓慢。

Table1 有大约。 38000 行。数据库在 Azure SQL v12.

上运行

Table:

CREATE TABLE [dbo].[Table1](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [KOD_ID] [int] NULL,
    [SYM] [nvarchar](20) NULL,
    [AN] [nvarchar](35) NULL,
    [A] [nvarchar](10) NULL,
    [B] [nvarchar](2) NULL,
    [C] [datetime] NULL,
    [D] [datetime] NULL,
 CONSTRAINT [PK_Table1] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
)
GO

CREATE NONCLUSTERED INDEX [IX_Table1] ON [dbo].[Table1]
(
    [KOD_ID] ASC,
    [SYM] ASC,
    [AN] 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)
GO

查询：

SELECT TOP 6000 *, RANK() OVER(ORDER BY KOD_ID ASC, SYM ASC, AN ASC) AS Rank#
FROM [dbo].[Table1]

SELECT TOP 7000 *, RANK() OVER(ORDER BY KOD_ID ASC, SYM ASC, AN ASC) AS Rank#
FROM [dbo].[Table1]

两个查询的执行计划

Answer 1

CREATE NONCLUSTERED INDEX [IX_Table1] ON [dbo].[Table1]
(
    [KOD_ID] ASC,
    [SYM] ASC,
    [AN] ASC
) INCLUDE ([A], [B], [C], [D]);

创建这样的覆盖索引，它应该扫描这个索引，很可能甚至不需要排序，因为它的数据已经在索引中排序。

您查询的要点是：

第一个方案有key lookup，尽量避免（key lookup是对每一行的额外扫描，因为索引没有）create covering indexes with INCLUDED columns
也要避免排序操作，它们对 SQL 服务器来说成本很高

如果您对索引重建没问题并且更喜欢读取而不是插入，考虑到这一点，这些可能是您 table 的备用 DDL，并且 KOD_ID、SYM、AN 不可为空：

如果需要ID保证唯一性：

CREATE TABLE [dbo].[Table1] (
    [KOD_ID] [int] NOT NULL
    , [SYM] [nvarchar](20) NOT NULL
    , [AN] [nvarchar](35) NOT NULL
    , [ID] [int] IDENTITY(1, 1) NOT NULL
    , [A] [nvarchar](10) NULL
    , [B] [nvarchar](2) NULL
    , [C] [datetime2] NULL
    , [D] [datetime2] NULL
    , CONSTRAINT [PK_Table1] PRIMARY KEY CLUSTERED ([KOD_ID], [SYM], [AN], [ID])
    );
GO

如果不需要ID保证唯一性：

CREATE TABLE [dbo].[Table1] (
    [KOD_ID] [int] NOT NULL
    , [SYM] [nvarchar](20) NOT NULL
    , [AN] [nvarchar](35) NOT NULL
    , [A] [nvarchar](10) NULL
    , [B] [nvarchar](2) NULL
    , [C] [datetime2] NULL
    , [D] [datetime2] NULL
    , CONSTRAINT [PK_Table1] PRIMARY KEY CLUSTERED ([KOD_ID], [SYM], [AN])
    );
GO

此外，请注意我使用 datetime2 而不是 datetime，这是 Microsoft 推荐的：https://docs.microsoft.com/en-us/sql/t-sql/data-types/datetime-transact-sql

Use the time, date, datetime2 and datetimeoffset data types for new work. These types align with the SQL Standard. They are more portable. time, datetime2 and datetimeoffset provide more seconds precision. datetimeoffset provides time zone support for globally deployed applications.

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