T-SQL复合索引足以查询列的子集吗？

Posted 2023-02-28

复合索引是否足以对列的子集进行查询？

CREATE TABLE [FILE_STATUS_HISTORY]
(
    [FILE_ID] [INT] NOT NULL,
    [STATUS_ID] [INT] NOT NULL,
    [TIMESTAMP_UTC] [DATETIME] NOT NULL,

    CONSTRAINT [PK_FILE_STATUS_HISTORY] 
        PRIMARY KEY CLUSTERED ([FILE_ID] ASC, [STATUS_ID] ASC)
) ON [PRIMARY]

CREATE UNIQUE NONCLUSTERED INDEX [IX_FILE_STATUS_HISTORY] 
ON [FILE_STATUS_HISTORY] ([FILE_ID] ASC,
                          [STATUS_ID] ASC,
                          [TIMESTAMP_UTC] ASC) ON [PRIMARY]
GO


SELECT TOP (1) *
FROM [FILE_STATUS_HISTORY]
WHERE [FILE_ID] = 382748
ORDER BY [TIMESTAMP_UTC] DESC

答案

实际执行计划将显示查询优化器是否同意。

covering index也将Status_Id作为included column，以便索引可以在一次查找中满足整个查询。