[英]why this is an index scan and not an index seek
已在dw_assesment_details
和dw_assesment_details_id
表上創建了聚集索引
/* 6 minutes */
CREATE CLUSTERED INDEX [Ix_DW_ASSESSMENT_DETAILS_qid_QNO_TmpverName]
ON [dbo].[DW_ASSESSMENT_DETAILS_QUESTION_ID]
(
[TEMPLATENAME] ASC,
[TEMPLATEVERSION] ASC,
[QUESTION_NO] 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
/* 9 minutes */
CREATE CLUSTERED INDEX [Ix_DW_ASSESSMENT_DETAILS_QNO_TmpverName]
ON [dbo].[DW_ASSESSMENT_DETAILS]
(
[TEMPLATENAME] ASC,
[TEMPLATEVERSION] ASC,
[QUESTION_NO] 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)
SELECT
[GETQUESTIONID],
dw.[TEMPLATENAME], dw.[TEMPLATEVERSION],
dw.[QUESTION_NO]
FROM
[QIS_DW].[dbo].[DW_ASSESSMENT_DETAILS] dw
INNER JOIN
[QIS_DW].[dbo].[DW_ASSESSMENT_DETAILS_QUESTION_ID] id ON dw.TEMPLATENAME = id.TEMPLATENAME
AND dw.TEMPLATEVERSION = id.TEMPLATEVERSION
AND dw.QUESTION_NO = id.QUESTION_NO
但是上面的選擇查詢正在使用索引掃描-而不是索引查找。 改用索引查找怎么辦?
性能調整專家有什么建議嗎?
改用索引查找怎么辦?
如果您想嘗試使SQL Server優化器的性能超越智能,則可以指定LOOP JOIN
查詢提示。 我希望MERGE JOIN
的計划在許多行中都能表現得更好。
請注意,存儲引擎還可以在大型掃描期間執行異步預讀,以將數據預取到內存中,以便查詢可以使用它,而不必等待從存儲中讀取數據。 返回幾行的搜索不會發生預讀。
請嘗試以下查詢,以查看您的環境是否屬於這種情況。
SET STATISTICS IO ON;
SET STATISTICS TIME ON;
GO
SELECT
[GETQUESTIONID],
dw.[TEMPLATENAME], dw.[TEMPLATEVERSION],
dw.[QUESTION_NO]
FROM
[QIS_DW].[dbo].[DW_ASSESSMENT_DETAILS] dw
INNER JOIN
[QIS_DW].[dbo].[DW_ASSESSMENT_DETAILS_QUESTION_ID] id ON dw.TEMPLATENAME = id.TEMPLATENAME
AND dw.TEMPLATEVERSION = id.TEMPLATEVERSION
AND dw.QUESTION_NO = id.QUESTION_NO;
GO
SELECT
[GETQUESTIONID],
dw.[TEMPLATENAME], dw.[TEMPLATEVERSION],
dw.[QUESTION_NO]
FROM
[QIS_DW].[dbo].[DW_ASSESSMENT_DETAILS] dw
INNER JOIN
[QIS_DW].[dbo].[DW_ASSESSMENT_DETAILS_QUESTION_ID] id ON dw.TEMPLATENAME = id.TEMPLATENAME
AND dw.TEMPLATEVERSION = id.TEMPLATEVERSION
AND dw.QUESTION_NO = id.QUESTION_NO
OPTION(LOOP JOIN);
GO
在SQL Server中,對群集或非群集索引表的搜索更改索引掃描,直到我們沒有在Where子句中添加條件為止。
Where子句將使Index尋求執行計划。 如果沒有Where子句,它將充當索引掃描來搜索整個表數據。 這就是索引掃描總是很慢的原因。
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