Considérez la requête ci-dessous:
CREATE PROC dbo.GetPage @orderid AS INT = 0, -- anchor sort key
@pagesize AS BIGINT = 25
AS
SELECT
TOP (@pagesize) orderid, orderdate, custid, empid
FROM dbo.Orders WHERE orderid > @orderid ORDER BY orderid;
exec GetPage 25,25
SET STATISTICS IO pour la requête ci-dessus retournée:
(25 row(s) affected)
Table 'Orders'. Scan count 1, logical reads 87, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Les explications d'Itzik Ben-Gan dans son livre sont les suivantes:
Les coûts d'E / S impliqués dans l'exécution du plan de requête sont les suivants:
- Cherchez la feuille d'index: 3 lectures (l'index a trois niveaux).
- Balayage de plage de 25 lignes: 0 à 1 lectures (des centaines de lignes tiennent dans une page).
- Prélecture de boucles imbriquées utilisée pour optimiser les recherches: 9 lectures (mesurée en désactivant la prélecture avec l'indicateur de trace 8744)
- 25 recherches clés: 75 lectures
Plan de requête
Maintenant, ma question est, puisque les boucles imbriquées effectuent une recherche de clé une fois pour chaque ligne renvoyée par la recherche, les lectures de recherche doivent-elles être 25 * 3: 75, les mêmes que les recherches de clés?
Plan de requête 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="1.4" Build="13.0.900.73" xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan">
<BatchSequence>
<Batch>
<Statements>
<StmtSimple StatementCompId="3" StatementEstRows="25" StatementId="1" StatementOptmLevel="FULL" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" CardinalityEstimationModelVersion="130" StatementSubTreeCost="0.0887816" StatementText=" CREATE PROC dbo.GetPage @orderid AS INT = 0, -- anchor sort key 
 @pagesize AS BIGINT = 25 
 AS
SELECT TOP (@pagesize) orderid, orderdate, custid, empid FROM dbo.Orders WHERE orderid > @orderid ORDER BY orderid" StatementType="SELECT" QueryHash="0x48DC1D1D4649B914" QueryPlanHash="0x8FDC055F05E0E93C" RetrievedFromCache="true" SecurityPolicyApplied="false">
<StatementSetOptions ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="true" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="true" />
<QueryPlan CachedPlanSize="32" CompileTime="2" CompileCPU="2" CompileMemory="208">
<MemoryGrantInfo SerialRequiredMemory="0" SerialDesiredMemory="0" />
<OptimizerHardwareDependentProperties EstimatedAvailableMemoryGrant="52428" EstimatedPagesCached="13107" EstimatedAvailableDegreeOfParallelism="2" />
<RelOp AvgRowSize="29" EstimateCPU="2.5E-06" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="25" LogicalOp="Top" NodeId="0" Parallel="false" PhysicalOp="Top" EstimatedTotalSubtreeCost="0.0887816">
<OutputList>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderid" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="custid" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="empid" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderdate" />
</OutputList>
<Top RowCount="false" IsPercent="false" WithTies="false">
<TopExpression>
<ScalarOperator ScalarString="[@pagesize]">
<Identifier>
<ColumnReference Column="@pagesize" />
</Identifier>
</ScalarOperator>
</TopExpression>
<RelOp AvgRowSize="29" EstimateCPU="4.1799" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="25" LogicalOp="Inner Join" NodeId="1" Parallel="false" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="0.0887791">
<OutputList>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderid" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="custid" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="empid" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderdate" />
</OutputList>
<NestedLoops Optimized="false" WithOrderedPrefetch="true">
<OuterReferences>
<ColumnReference Column="Uniq1001" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderdate" />
<ColumnReference Column="Expr1003" />
</OuterReferences>
<RelOp AvgRowSize="18" EstimateCPU="1.10013" EstimateIO="1.92683" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="25" LogicalOp="Index Seek" NodeId="3" Parallel="false" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.00335567" TableCardinality="1000000">
<OutputList>
<ColumnReference Column="Uniq1001" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderid" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderdate" />
</OutputList>
<IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" ForceSeek="false" ForceScan="false" NoExpandHint="false" Storage="RowStore">
<DefinedValues>
<DefinedValue>
<ColumnReference Column="Uniq1001" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderid" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderdate" />
</DefinedValue>
</DefinedValues>
<Object Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Index="[PK_Orders]" IndexKind="NonClustered" Storage="RowStore" />
<SeekPredicates>
<SeekPredicateNew>
<SeekKeys>
<StartRange ScanType="GT">
<RangeColumns>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderid" />
</RangeColumns>
<RangeExpressions>
<ScalarOperator ScalarString="[@orderid]">
<Identifier>
<ColumnReference Column="@orderid" />
</Identifier>
</ScalarOperator>
</RangeExpressions>
</StartRange>
</SeekKeys>
</SeekPredicateNew>
</SeekPredicates>
</IndexScan>
</RelOp>
<RelOp AvgRowSize="22" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="25" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="1" LogicalOp="Clustered Index Seek" NodeId="5" Parallel="false" PhysicalOp="Clustered Index Seek" EstimatedTotalSubtreeCost="0.0853189" TableCardinality="1000000">
<OutputList>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="custid" />
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="empid" />
</OutputList>
<IndexScan Lookup="true" Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" ForceSeek="false" ForceScan="false" NoExpandHint="false" Storage="RowStore">
<DefinedValues>
<DefinedValue>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="custid" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="empid" />
</DefinedValue>
</DefinedValues>
<Object Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Index="[idx_cl_od]" TableReferenceId="-1" IndexKind="Clustered" Storage="RowStore" />
<SeekPredicates>
<SeekPredicateNew>
<SeekKeys>
<Prefix ScanType="EQ">
<RangeColumns>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderdate" />
<ColumnReference Column="Uniq1001" />
</RangeColumns>
<RangeExpressions>
<ScalarOperator ScalarString="[PerformanceV3].[dbo].[Orders].[orderdate]">
<Identifier>
<ColumnReference Database="[PerformanceV3]" Schema="[dbo]" Table="[Orders]" Column="orderdate" />
</Identifier>
</ScalarOperator>
<ScalarOperator ScalarString="[Uniq1001]">
<Identifier>
<ColumnReference Column="Uniq1001" />
</Identifier>
</ScalarOperator>
</RangeExpressions>
</Prefix>
</SeekKeys>
</SeekPredicateNew>
</SeekPredicates>
</IndexScan>
</RelOp>
</NestedLoops>
</RelOp>
</Top>
</RelOp>
<ParameterList>
<ColumnReference Column="@pagesize" ParameterCompiledValue="(25)" />
<ColumnReference Column="@orderid" ParameterCompiledValue="(25)" />
</ParameterList>
</QueryPlan>
</StmtSimple>
</Statements>
</Batch>
</BatchSequence>
</ShowPlanXML>
sql-server
execution-plan
database-internals
sql-server-2016
TheGameiswar
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