SQL联合所有查询都有3个联合所有查询。在查询中添加了文本列CAST(c.getQuestionId AS VARCHAR(300))后，查询执行时间发生了显着变化。正在使用的数据库是SQL SERVER 2014.性能调优专家，请帮忙。
在JcccustomersAssessmentProxy上创建了3个索引。这是一张有8000万条记录的表格。索引的显式使用仅在第3个查询中提及
（即（NOLOCK, INDEX=IX_AssessmentProxy_myJcAssessmentContext））
myJcAssessmentContext上有一个非聚集索引
getmycoordtoyplanver上有一个非聚集索引
getMyEventItem上有一个非聚集索引
SELECT ass.P_KEY
,ass.SS_CODE
,CAST(evp.EVENT_SSID AS VARCHAR(11)) AS EVENT_SSID
,CAST(QUESTIONNUMBER AS VARCHAR(3)) AS EVENT_NO
,CAST(ISNULL(CAST(TEMPLATEVERSION AS VARCHAR), 0) AS INT) AS TEMPLATEVERSION
,ISNULL(CAST(TEMPLATENAME AS VARCHAR(50)), ' ') AS TEMPLATENAME
,evp.ASS_DATE
,ISNULL(ANSWERNUMBER, 0) AS ANSWER_NO
,ass.CASE_SSID
,ass.RE_SSID
,ass.RE_DATE
,ass.EPISODE_SSID
,ass.[SERVICE]
,ass.SERVICE_DESC
,ass.TAM_KEY
,ass.PRv_KEY
,CAST(QUESTIONNUMBER AS VARCHAR(3)) AS QUESTION_NO
,ISNULL(CONVERT(VARCHAR(100), REPLACE(REPLACE(CAST(QUESTIONTEXT AS VARCHAR(650)), char(10), ''), char(13), ' ')), ' ') AS QUESTION_TEXT
,ISNULL(CAST(ANSWER AS VARCHAR(125)), ' ') AS ANSWER_TEXT
,a.MyAssessment AS ASSESSMENT_SSID
-----------------------------------------------------------------------------------------
,CAST(RIGHT(a.getMyCoordtoyPlanVer, 10) AS INT) AS toy_PLAN_VERSION_SSID
,CAST(RIGHT(a.myJcAssessmentContext, 10) AS INT) AS CONTEXT_FORM_SSID
-----------------------------------------------------------------------------------------
,ass.RECORD_DATE
**,CAST(a.getQuestionId AS VARCHAR(300) ) AS QUESTION_ID /*NEW Text Column ,this remove the parallelism*/**
FROM SourceFeed.dbo.JcccustomersAssessmentProxy AS a WITH(NOLOCK)
INNER JOIN
TEMP_DATABASE.dbo.jsystemReport_AllEventItems_AllPersons evp WITH(NOLOCK)
ON a.getMyEventItem = evp.oid
INNER JOIN
TEMP_DATABASE.dbo.jsystemAssessment ass WITH(NOLOCK)
ON evp.getcustomersId = ass.customersID
AND evp.EVENT_SSID = ass.EVENTID
UNION ALL
SELECT ass.P_KEY
,ass.SS_CODE
,CAST(csp.ENT_SSID AS VARCHAR(11)) AS EVENT_SSID
,CAST(jcc.QUESTIONNUMBER AS VARCHAR(3)) AS EVENT_NO
,CAST(ISNULL(CAST(CAST(jcc.TEMPLATEVERSION AS VARCHAR) AS VARCHAR), 0) AS INT) AS TEMPLATEVERSION
,ISNULL(CAST(jcc.TEMPLATENAME AS VARCHAR(50)), ' ') AS TEMPLATENAME
,csp.ASSESSMENT_DATE
,ISNULL(jcc.ANSWERNUMBER, 0) AS ANSWER_NO
,ass.CASE_SSID
,ass.REF_SSID
,ass.RE_DATE
,ass.EPISODE_SSID
,ass.[SERVICE]
,ass.SERVICE_DESC
,ass.TEAM_KEY
,ass.PROVIDER_KEY
,CAST(ISNULL(jcc.QUESTIONNUMBER, 0) AS VARCHAR(3)) AS QUESTION_NO
,isnull(CONVERT(VARCHAR(100), replace(replace(CAST(jcc.QUESTIONTEXT AS VARCHAR(650)), char(10), ''), char(13), ' ')), ' ') AS QUESTION_TEXT
,ISNULL(CAST(jcc.ANSWER AS VARCHAR(125)), ' ') AS ANSWER_TEXT
,jcc.MyAssessment AS ASSESSMENT_SSID
-----------------------------------------------------------------------------------------
,CAST(RIGHT(jcc.getMyCoordtoyPlanVer, 10) AS INT) AS toy_PLAN_VERSION_SSID
,CAST(RIGHT(jcc.myJcAssessmentContext, 10) AS INT) AS CONTEXT_FORM_SSID
-----------------------------------------------------------------------------------------
,ass.RECORD_DATE
**,CAST(jcc.getQuestionId AS VARCHAR(300) ) AS QUESTION_ID /*NEW Text Column ,this remove the parallelism*/**
FROM TEMP_DATABASE.dbo.jsystemReport_toySpell csp WITH(NOLOCK)
INNER JOIN
TEMP_DATABASE.dbo.jsystemAssessment ass WITH(NOLOCK)
ON csp.getcustomersId = ass.customersID
AND csp.EVENT_SSID = ass.EVENTID
LEFT OUTER JOIN
SourceFeed.dbo.JcccustomersAssessmentProxy jcc WITH(NOLOCK)
ON csp.OID = jcc.getmycoordtoyplanver
WHERE NOT EXISTS (SELECT tp1.OID
FROM TEMP_DATABASE.dbo.AssessmentTransferPart1 tp1
WHERE tp1.OID = jcc.OID)
UNION ALL
SELECT ass.P_KEY
,ass.SS_CODE
,CAST(a.EVENT_SSID AS VARCHAR(11)) AS EVENT_SSID
,CAST(CAST(C.QUESTIONNUMBER AS INT) AS CHAR(3)) AS EVENT_NO
,CAST(ISNULL(CAST(C.TEMPLATEVERSION AS VARCHAR), 0) AS INT) AS TEMPLATEVERSION
,ISNULL(CAST(C.TEMPLATENAME AS VARCHAR(50)), ' ') AS TEMPLATENAME
,a.ASSESSMENT_DATE
,ISNULL(C.ANSWERNUMBER, 0) AS ANSWER_NO
,ass.CASE_SSID
,ass.REL_SSID
,ass.REAL_DATE
,ass.EPISODE_SSID
,ass.[SERVICE]
,ass.SERVICE_DESC
,ass.TEAM_KEY
,ass.PROVIDER_KEY
,CAST(ISNULL(CAST(C.QUESTIONNUMBER AS INT), 0) AS VARCHAR(3)) AS QUESTION_NO
,isnull(CONVERT(VARCHAR(100), CAST(C.QUESTIONTEXT AS VARCHAR(650))), ' ') AS QUESTION_TEXT
,ISNULL(CAST(C.ANSWER AS VARCHAR(125)), ' ') AS ANSWER_TEXT
,a.MyAssessment AS ASSESSMENT_SSID
-----------------------------------------------------------------------------------------
,CAST(RIGHT(C.getMyCoordtoyPlanVer, 10) AS INT) AS toy_PLAN_VERSION_SSID
,CAST(RIGHT(C.myJcAssessmentContext, 10) AS INT) AS CONTEXT_FORM_SSID
-----------------------------------------------------------------------------------------
,ass.RECORD_DATE
**,CAST(c.getQuestionId AS VARCHAR(300) )** AS QUESTION_ID /*NEW Text Column ,this remove the parallelism*/
FROM SourceFeed.dbo.toyClusterReviewAssessment B WITH(NOLOCK)
INNER JOIN
SourceFeed.dbo.JcccustomersAssessmentProxy C WITH(NOLOCK, INDEX=IX_AssessmentProxy_myJcAssessmentContext)
ON B.myJCJccUserFormContext = C.myJcAssessmentContext
INNER JOIN
SourceFeed.dbo.JcccustomersAssessmentScoreProxy D WITH(NOLOCK)
ON B.myJCJccUserFormContext = D.myJcAssessmentContext
INNER JOIN
TEMP_DATABASE.dbo.jsystemReport_toyClusterReviewEvent_AllPersons A WITH(NOLOCK)
ON a.myAssessment = B.oid
INNER JOIN
TEMP_DATABASE.dbo.jsystemAssessment ass WITH(NOLOCK)
ON a.getcustomersId = ass.customersID
AND a.EVENT_SSID = ass.EVENTID
WHERE NOT EXISTS (SELECT OID
FROM (SELECT tp1.OID
FROM TEMP_DATABASE.dbo.AssessmentTransferPart1 tp1
UNION ALL
SELECT tp2.OID
FROM TEMP_DATABASE.dbo.AssessmentTransferPart2 tp2) jcc
WHERE jcc.OID = C.OID)
这是link to the actual execution plan。
在JccClientAssessmentProxy jcc WITH上添加索引后（NOLOCK，INDEX = IX_AssessmentProxy_GETMYCOORDCAREPLANVER）
这是the new execution plan。
答案 0 :(得分：1)
有问题的部分是：
,CAST(a.getQuestionId AS VARCHAR(300)) AS QUESTION_ID
FROM cnlPjccR_Report.dbo.JccClientAssessmentProxy AS a
WITH(NOLOCK,INDEX=IX_AssessmentProxy_getMyEventItem)
INNER JOIN ...
并且问题会在两个UNION ALL子查询中重复出现。
您的执行计划显示费用 Key lookups 。
第二种方法是查看是否可以创建“覆盖索引” 满足整个查询或至少消除键查找。 A. “覆盖索引”只是一个包含所有内容的非聚集索引 列需要满足整个查询或在我们的情况下， 消除了对键查找操作的需要。得到的一个挑战是 生成密钥查找的列的列表。你可以这样做 在SQL Server Management Studio（SSMS）中右键单击该键 查找运算符，然后选择“属性”。然后找到输出 在“属性”窗口中列出行，然后单击省略号按钮。 这将打开一个窗口（见下文），其中包含所有列的列表 密钥查找正在寻找。您可以使用此列表来帮助您 决定是否以及如何创建索引以“覆盖”查询或键 查找。
在你的情况下，在后面的列中，应该包含以创建覆盖索引：
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].answer;
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].answerNumber;
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].getMyCoordCarePlanVer;
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].getQuestionId;
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].myAssessment;
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].myJcAssessmentContext;
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].questionNumber;
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].questionText;
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].templateName;
[cnlPjccR_Report].[dbo].[JccClientAssessmentProxy].templateVersion
您的索引可能不包含getQuestionId列，并且该事实在您的查询执行计划中创建了较慢的Key lookup或RID lookup。
请参阅also here或this answer。