SQL Server Query Plans, Elapsed Time is Lower, but Subtree Cost is Higher

Question

SQL Server Query Plans, Elapsed Time is Lower, but Subtree Cost is Higher

Kuy*_*nda 3 sql-server sql-execution-plan

Why is the elapsed execution time of first of the two queries below lower, while the estimated subtree cost is higher?

Estimated subtree cost is only a guideline for gauging query plan performance in conjunction with other query performance indicators, like response time, but I'm surprised by the existance of an inverse relationship between cost and time when comparing two similar queries side by side.

I've created an example database to illustrate this problem and included the resulting XML query plans for both queries. I realize the existence of the StateID as a foreign key in both the County and Town tables is bad form, but it was the easiest way for me to illustrate this phenomenon.

I'm not just asking "which is more important, subtree cost or response time." I want to understand how an inverse relationship can exist between query plan subtree cost and response time for two similar queries, so that I can make more informed decisions going forward.

Please note that the two queries below produce the exact same result set.

Thanks!

CREATE TABLE dbo.County (StateID INT, CountyID INT)
CREATE TABLE dbo.Town (StateID INT, CountyID INT, TownID INT)

DECLARE @State TABLE (StateID INT, PRIMARY KEY CLUSTERED (StateID))
DECLARE @County TABLE (CountyID INT, PRIMARY KEY CLUSTERED (CountyID))

INSERT INTO @State
VALUES (27)

INSERT INTO @County
SELECT DISTINCT
    c.CountyID
FROM dbo.County c
JOIN @State s ON c.StateID = s.StateID

-- low time, high cost query
SELECT DISTINCT
    t.CountyID,
    t.TownID
FROM dbo.Town t
JOIN @State s
    ON t.StateID = s.StateID

-- low cost, high time query
SELECT DISTINCT
    t.CountyID,
    t.TownID
FROM dbo.Town t
JOIN @State s
    ON t.StateID = s.StateID
JOIN @County c
    ON t.CountyID = c.CountyID

Run Code Online (Sandbox Code Playgroud)

The following query plan is for the query that had an estimated subtree cost of 6.06 and an elapsed time of 1114 milliseconds.

<StmtSimple StatementCompId="6" StatementEstRows="273495" StatementId="3" StatementOptmLevel="FULL" StatementSubTreeCost="6.06304" StatementText="SELECT DISTINCT&#xD;&#xA;&#x9;t.CountyID,&#xD;&#xA;&#x9;t.TownID&#xD;&#xA;FROM dbo.Town t&#xD;&#xA;JOIN @State s&#xD;&#xA;&#x9;ON t.StateID = s.StateID&#xD;&#xA;&#xD;&#xA;-- low cost, high time query&#xD;" StatementType="SELECT" QueryHash="0x9347C19165C31DBF" QueryPlanHash="0x98EE57C66D8B347A">
          <StatementSetOptions ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="true" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="true" />
          <QueryPlan DegreeOfParallelism="2" MemoryGrant="28144" CachedPlanSize="32" CompileTime="8" CompileCPU="8" CompileMemory="312">
            <RelOp AvgRowSize="15" EstimateCPU="0.388488" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Gather Streams" NodeId="0" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="6.06304">
              <OutputList>
                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
              </OutputList>
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRows="13027" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <Parallelism>
                <RelOp AvgRowSize="15" EstimateCPU="3.79725" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Aggregate" NodeId="1" Parallel="true" PhysicalOp="Hash Match" EstimatedTotalSubtreeCost="5.67455">
                  <OutputList>
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                  </OutputList>
                  <MemoryFractions Input="0.998723" Output="1" />
                  <RunTimeInformation>
                    <RunTimeCountersPerThread Thread="2" ActualRows="6515" ActualEndOfScans="1" ActualExecutions="1" />
                    <RunTimeCountersPerThread Thread="1" ActualRows="6512" ActualEndOfScans="1" ActualExecutions="1" />
                    <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                  </RunTimeInformation>
                  <Hash>
                    <DefinedTowns />
                    <HashKeysBuild>
                      <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                      <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                    </HashKeysBuild>
                    <BuildResidual>
                      <ScalarOperator ScalarString="[Locality].[dbo].[Town].[CountyID] as [t].[CountyID] = [Locality].[dbo].[Town].[CountyID] as [t].[CountyID] AND [Locality].[dbo].[Town].[TownID] as [t].[TownID] = [Locality].[dbo].[Town].[TownID] as [t].[TownID]">
                        <Logical Operation="AND">
                          <ScalarOperator>
                            <Compare CompareOp="IS">
                              <ScalarOperator>
                                <Identifier>
                                  <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                </Identifier>
                              </ScalarOperator>
                              <ScalarOperator>
                                <Identifier>
                                  <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                </Identifier>
                              </ScalarOperator>
                            </Compare>
                          </ScalarOperator>
                          <ScalarOperator>
                            <Compare CompareOp="IS">
                              <ScalarOperator>
                                <Identifier>
                                  <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                                </Identifier>
                              </ScalarOperator>
                              <ScalarOperator>
                                <Identifier>
                                  <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                                </Identifier>
                              </ScalarOperator>
                            </Compare>
                          </ScalarOperator>
                        </Logical>
                      </ScalarOperator>
                    </BuildResidual>
                    <RelOp AvgRowSize="15" EstimateCPU="0.494228" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Repartition Streams" NodeId="2" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="1.8773">
                      <OutputList>
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="2" ActualRows="1017328" ActualEndOfScans="1" ActualExecutions="1" />
                        <RunTimeCountersPerThread Thread="1" ActualRows="1093191" ActualEndOfScans="1" ActualExecutions="1" />
                        <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                      </RunTimeInformation>
                      <Parallelism PartitioningType="Hash">
                        <PartitionColumns>
                          <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                          <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                        </PartitionColumns>
                        <RelOp AvgRowSize="15" EstimateCPU="0.571604" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Inner Join" NodeId="3" Parallel="true" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="1.38307">
                          <OutputList>
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="2" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" />
                            <RunTimeCountersPerThread Thread="1" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
                            <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                          </RunTimeInformation>
                          <NestedLoops Optimized="true">
                            <OuterReferences>
                              <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                            </OuterReferences>
                            <RelOp AvgRowSize="11" EstimateCPU="0.0285019" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Distribute Streams" NodeId="5" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="0.031785">
                              <OutputList>
                                <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                              </OutputList>
                              <RunTimeInformation>
                                <RunTimeCountersPerThread Thread="2" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" />
                                <RunTimeCountersPerThread Thread="1" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
                                <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                              </RunTimeInformation>
                              <Parallelism PartitioningType="RoundRobin">
                                <RelOp AvgRowSize="11" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Clustered Index Scan" NodeId="6" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="0">
                                  <OutputList>
                                    <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                                  </OutputList>
                                  <RunTimeInformation>
                                    <RunTimeCountersPerThread Thread="1" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
                                    <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                                  </RunTimeInformation>
                                  <IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                                    <DefinedTowns>
                                      <DefinedTown>
                                        <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                                      </DefinedTown>
                                    </DefinedTowns>
                                    <Object Table="[@State]" Index="[PK__#3AB788A__C3BA3B5A3C9FD11A]" Alias="[s]" />
                                  </IndexScan>
                                </RelOp>
                              </Parallelism>
                            </RelOp>
                            <RelOp AvgRowSize="15" EstimateCPU="0.301001" EstimateIO="0.478681" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Index Seek" NodeId="7" Parallel="true" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.779682" TableCardinality="6291320">
                              <OutputList>
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                              </OutputList>
                              <RunTimeInformation>
                                <RunTimeCountersPerThread Thread="2" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                                <RunTimeCountersPerThread Thread="1" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
                                <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                              </RunTimeInformation>
                              <IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" ForceSeek="false" NoExpandHint="false">
                                <DefinedTowns>
                                  <DefinedTown>
                                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                  </DefinedTown>
                                  <DefinedTown>
                                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                                  </DefinedTown>
                                </DefinedTowns>
                                <Object Database="[Locality]" Schema="[dbo]" Table="[Town]" Index="[NCX_Town_StateID_CountyID_TownID]" Alias="[t]" IndexKind="NonClustered" />
                                <SeekPredicates>
                                  <SeekPredicateNew>
                                    <SeekKeys>
                                      <Prefix ScanType="EQ">
                                        <RangeColumns>
                                          <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                                        </RangeColumns>
                                        <RangeExpressions>
                                          <ScalarOperator ScalarString="@State.[StateID] as [s].[StateID]">
                                            <Identifier>
                                              <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                                            </Identifier>
                                          </ScalarOperator>
                                        </RangeExpressions>
                                      </Prefix>
                                    </SeekKeys>
                                  </SeekPredicateNew>
                                </SeekPredicates>
                              </IndexScan>
                            </RelOp>
                          </NestedLoops>
                        </RelOp>
                      </Parallelism>
                    </RelOp>
                  </Hash>
                </RelOp>
              </Parallelism>
            </RelOp>
          </QueryPlan>
        </StmtSimple>

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以下查询计划适用于估计子树成本为0.14且经过时间为4614毫秒的查询.

<StmtSimple StatementCompId="7" StatementEstRows="339.324" StatementId="4" StatementOptmLevel="FULL" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" StatementSubTreeCost="0.142839" StatementText="SELECT DISTINCT&#xD;&#xA;&#x9;t.CountyID,&#xD;&#xA;&#x9;t.TownID&#xD;&#xA;FROM dbo.Town t&#xD;&#xA;JOIN @State s&#xD;&#xA;&#x9;ON t.StateID = s.StateID&#xD;&#xA;JOIN @County c&#xD;&#xA;&#x9;ON t.CountyID = c.CountyID&#xD;&#xA;&#xD;" StatementType="SELECT" QueryHash="0xF7B13FA8059B141C" QueryPlanHash="0x31CEE9DAF12E77A5">
          <StatementSetOptions ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="true" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="true" />
          <QueryPlan DegreeOfParallelism="1" MemoryGrant="1584" CachedPlanSize="40" CompileTime="5" CompileCPU="5" CompileMemory="376">
            <RelOp AvgRowSize="15" EstimateCPU="0.00454996" EstimateIO="0.0112613" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="339.324" LogicalOp="Distinct Sort" NodeId="0" Parallel="false" PhysicalOp="Sort" EstimatedTotalSubtreeCost="0.142839">
              <OutputList>
                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
              </OutputList>
              <MemoryFractions Input="1" Output="1" />
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRebinds="1" ActualRewinds="0" ActualRows="13027" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <Sort Distinct="true">
                <OrderBy>
                  <OrderByColumn Ascending="true">
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                  </OrderByColumn>
                  <OrderByColumn Ascending="true">
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                  </OrderByColumn>
                </OrderBy>
                <RelOp AvgRowSize="15" EstimateCPU="0.0571741" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="339.324" LogicalOp="Inner Join" NodeId="1" Parallel="false" PhysicalOp="Hash Match" EstimatedTotalSubtreeCost="0.127028">
                  <OutputList>
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                    <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                  </OutputList>
                  <MemoryFractions Input="0" Output="0" />
                  <RunTimeInformation>
                    <RunTimeCountersPerThread Thread="0" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
                  </RunTimeInformation>
                  <Hash>
                    <DefinedTowns />
                    <HashKeysBuild>
                      <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                    </HashKeysBuild>
                    <HashKeysProbe>
                      <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                    </HashKeysProbe>
                    <RelOp AvgRowSize="11" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Clustered Index Scan" NodeId="2" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="0">
                      <OutputList>
                        <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="0" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
                      </RunTimeInformation>
                      <IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                        <DefinedTowns>
                          <DefinedTown>
                            <ColumnReference Table="@State" Alias="[s]" Column="StateID" />
                          </DefinedTown>
                        </DefinedTowns>
                        <Object Table="[@State]" Index="[PK__#3AB788A__C3BA3B5A3C9FD11A]" Alias="[s]" />
                      </IndexScan>
                    </RelOp>
                    <RelOp AvgRowSize="19" EstimateCPU="0.0357739" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="8558.34" LogicalOp="Inner Join" NodeId="3" Parallel="false" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="0.066568">
                      <OutputList>
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                        <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="0" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
                      </RunTimeInformation>
                      <NestedLoops Optimized="false">
                        <OuterReferences>
                          <ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
                        </OuterReferences>
                        <RelOp AvgRowSize="11" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Clustered Index Scan" NodeId="4" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="0">
                          <OutputList>
                            <ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="0" ActualRows="137" ActualEndOfScans="1" ActualExecutions="1" />
                          </RunTimeInformation>
                          <IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                            <DefinedTowns>
                              <DefinedTown>
                                <ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
                              </DefinedTown>
                            </DefinedTowns>
                            <Object Table="[@County]" Index="[PK__#3E88198__B68F9DF7407061FE]" Alias="[c]" />
                          </IndexScan>
                        </RelOp>
                        <RelOp AvgRowSize="19" EstimateCPU="0.00957118" EstimateIO="0.0179398" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="8558.34" LogicalOp="Index Seek" NodeId="5" Parallel="false" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.027511" TableCardinality="6291320">
                          <OutputList>
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                            <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="0" ActualRows="2110519" ActualEndOfScans="137" ActualExecutions="137" />
                          </RunTimeInformation>
                          <IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" ForceSeek="false" NoExpandHint="false">
                            <DefinedTowns>
                              <DefinedTown>
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
                              </DefinedTown>
                              <DefinedTown>
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                              </DefinedTown>
                              <DefinedTown>
                                <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
                              </DefinedTown>
                            </DefinedTowns>
                            <Object Database="[Locality]" Schema="[dbo]" Table="[Town]" Index="[NCX_Town_CountyID_inc_StateID_TownID]" Alias="[t]" IndexKind="NonClustered" />
                            <SeekPredicates>
                              <SeekPredicateNew>
                                <SeekKeys>
                                  <Prefix ScanType="EQ">
                                    <RangeColumns>
                                      <ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
                                    </RangeColumns>
                                    <RangeExpressions>
                                      <ScalarOperator ScalarString="@County.[CountyID] as [c].[CountyID]">
                                        <Identifier>
                                          <ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
                                        </Identifier>
                                      </ScalarOperator>
                                    </RangeExpressions>
                                  </Prefix>
                                </SeekKeys>
                              </SeekPredicateNew>
                            </SeekPredicates>
                          </IndexScan>
                        </RelOp>
                      </NestedLoops>
                    </RelOp>
                  </Hash>
                </RelOp>
              </Sort>
            </RelOp>
          </QueryPlan>
        </StmtSimple>

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Answer 1

Aar*_*iel 7

虽然我没有深入研究您的查询计划,但我有建议和建议的解释.建议很简单:不要将表变量用于行数超过两个数量级(100行)的表.可能的解释是,根据Microsoft关于表数据类型的文章,它们的性能可能非常差,并且生成的查询计划可能不准确或效率低下:

最佳做法

不要使用表变量来存储大量数据(超过100行).当表变量包含大量数据时,计划选择可能不是最佳的或稳定的.考虑重写此类查询以使用临时表或使用USE PLAN查询提示来确保优化程序使用适用于您的方案的现有查询计划.

和:

限制和限制

SQL Server优化器的基于开销的推理模型不支持表变量.因此,当需要基于成本的选择来实现有效的查询计划时,不应使用它们.当需要基于成本的选择时,首选临时表.这通常包括具有连接,并行性决策和索引选择选择的查询.

修改表变量的查询不会生成并行查询执行计划.当修改非常大的表变量或复杂查询中的表变量时,性能会受到影响.在这些情况下,请考虑使用临时表.有关更多信息,请参见CREATE TABLE(Transact-SQL).读取表变量而不修改它们的查询仍然可以并行化.

...

在实践中,我发现使用表变量会排除并行查询计划的生成,并且会损害除最简单,最不复杂的表变量之外的所有变量的性能.

相反,我发现几乎在每种情况下临时表都更具性能,并且只要你小心地及时清理自己(或使用短期会话以允许服务器清除临时表)然后你获得更好的表现.

至于为什么第二个查询计划的成本较低但执行时间较长？可能有很多很多县(1000s？10000s？),生成的查询计划假设有一个很小的数字(100s).查询优化器认为 @County可能少于100行,因此它可能认为结果集非常具体,因为城镇列表加入了一个<100个条目的列表,然后加入第二个<100的列表条目很可能少于100行.

实际上,每个城镇都有一个县,因此连接不是很有选择性,并且使用连接类型的不良选择.在每种情况下,嵌套循环用于最后一次连接,但嵌套循环连接对于大型排序数据效率极低.这可能是问题的症结所在.

使用临时表而不是表变量重新运行查询,我猜你将获得具有非常不同的性能特征的非常不同的查询计划.

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