我无法理解为什么在 2 个表上执行内部联接的看似简单的查询会导致估计的行数为 1，而实际的行数几乎为 200 万。我没有看到缺少索引的任何问题，并且 98% 的查询成本是作为索引查找发生的。当他们查询运行时，我没有看到任何 I/O 或 CPU 危险信号，这大约需要 12 秒。在这里添加统计信息似乎没有意义，因为查询只是加入了 id，并且两个表都将这些 id 上的聚集索引作为主键。
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。我假设估计行和实际行之间的差异至少会导致查询性能欠佳。有趣的是，用 LEFT OUTER JOIN 代替 INNER JOIN 会导致大致相同的性能，即使它确实修复了 Estimated Rows 是正确的。
询问：
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>