我很难理解为什么一个看似简单的查询（对2个表进行内部联接）会导致估计的行数为1，而实际却接近200万。我看不到缺少索引的任何问题，而98％的查询成本是作为索引查找发生的。我在查询运行时看不到任何I / O或CPU红色标志，这大约需要12秒。在这里添加统计信息似乎没有意义，因为查询只是在id上联接，并且两个表在这些id上都有聚集索引作为主键。
此查询是遇到相同问题的较大查询的简化版本，但我将其归结为遇到相同问题的简化查询。我认为估计行与实际行之间的差异至少会导致次优查询性能。有趣的是，用左外连接代替内连接会导致大致相同的性能，即使它确实将估计行固定为正确。
查询：
SELECT StationId, Readings.Power, Readings.TimeCovered
FROM Readings
INNER JOIN Stations ON Readings.StationId = Stations.Id
实际计划XML：
<?xml version="1.0" encoding="utf-16"?>
<ShowPlanXML xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" Version="1.1" Build="10.50.1600.1" xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan">
<BatchSequence>
<Batch>
<Statements>
<StmtSimple StatementCompId="1" StatementEstRows="1" StatementId="1" StatementOptmLevel="FULL" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" StatementSubTreeCost="0.388499" StatementText="SELECT StationId,
[Readings].[Power], [Readings].[TimeCovered]
FROM [dbo].[Readings]
INNER JOIN [dbo].[Stations] ON [Readings].[StationId] = [Stations].[Id]" StatementType="SELECT" QueryHash="0x540DF2384788314E" QueryPlanHash="0x7546B31B38AC8153">
<StatementSetOptions ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" ARITHABORT="false" CONCAT_NULL_YIELDS_NULL="true" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="true" />
<QueryPlan DegreeOfParallelism="1" CachedPlanSize="24" CompileTime="3" CompileCPU="3" CompileMemory="224">
<RelOp AvgRowSize="17" EstimateCPU="0.00118294" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Inner Join" NodeId="0" Parallel="false" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="0.388499">
<OutputList>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="StationId" />
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="Power" />
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="TimeCovered" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="1898419" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<NestedLoops Optimized="false">
<OuterReferences>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Stations]" Column="Id" />
</OuterReferences>
<RelOp AvgRowSize="11" EstimateCPU="0.0004683" EstimateIO="0.00608796" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="283" LogicalOp="Clustered Index Scan" NodeId="1" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.00655626" TableCardinality="283">
<OutputList>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Stations]" Column="Id" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="283" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
<DefinedValues>
<DefinedValue>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Stations]" Column="Id" />
</DefinedValue>
</DefinedValues>
<Object Database="[MyDB]" Schema="[dbo]" Table="[Stations]" Index="[PK_Stations]" IndexKind="Clustered" />
</IndexScan>
</RelOp>
<RelOp AvgRowSize="17" EstimateCPU="0.0012571" EstimateIO="0.025" EstimateRebinds="0" EstimateRewinds="282" EstimateRows="1" LogicalOp="Index Seek" NodeId="2" Parallel="false" Partitioned="true" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.380759" TableCardinality="4246720">
<OutputList>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="StationId" />
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="Power" />
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="TimeCovered" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="1898419" ActualEndOfScans="283" ActualExecutions="283" />
</RunTimeInformation>
<RunTimePartitionSummary>
<PartitionsAccessed PartitionCount="8">
<PartitionRange Start="1" End="8" />
</PartitionsAccessed>
</RunTimePartitionSummary>
<IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" ForceSeek="false" NoExpandHint="false">
<DefinedValues>
<DefinedValue>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="StationId" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="Power" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="TimeCovered" />
</DefinedValue>
</DefinedValues>
<Object Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Index="[IX_Readings_StationId_SecondsSinceEpoch]" IndexKind="NonClustered" />
<SeekPredicates>
<SeekPredicateNew>
<SeekKeys>
<StartRange ScanType="GE">
<RangeColumns>
<ColumnReference Column="PtnId1000" />
</RangeColumns>
<RangeExpressions>
<ScalarOperator ScalarString="(1)">
<Const ConstValue="(1)" />
</ScalarOperator>
</RangeExpressions>
</StartRange>
<EndRange ScanType="LE">
<RangeColumns>
<ColumnReference Column="PtnId1000" />
</RangeColumns>
<RangeExpressions>
<ScalarOperator ScalarString="(8)">
<Const ConstValue="(8)" />
</ScalarOperator>
</RangeExpressions>
</EndRange>
</SeekKeys>
<SeekKeys>
<Prefix ScanType="EQ">
<RangeColumns>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Readings]" Column="StationId" />
</RangeColumns>
<RangeExpressions>
<ScalarOperator ScalarString="[MyDB].[dbo].[Stations].[Id]">
<Identifier>
<ColumnReference Database="[MyDB]" Schema="[dbo]" Table="[Stations]" Column="Id" />
</Identifier>
</ScalarOperator>
</RangeExpressions>
</Prefix>
</SeekKeys>
</SeekPredicateNew>
</SeekPredicates>
</IndexScan>
</RelOp>
</NestedLoops>
</RelOp>
</QueryPlan>
</StmtSimple>
</Statements>
</Batch>
</BatchSequence>
</ShowPlanXML>
最佳答案
未更新的统计信息可能与此有关。尝试运行exec sp_updatestats，看看计划是否改变。另外，请确保您在SQL服务器上运行了一项维护作业，该作业会定期更新统计信息。