Forcer une bobine d'index
-
31-10-2019 - |
Question
Je sais que c'est quelque chose qui devrait être évité pour des raisons de performance, mais j'essaie de montrer une condition où elle apparaît comme une démo sur la façon de s'assurer qu'elle n'apparaît pas.
Cependant, je me retrouve avec un avertissement d'index manquant, mais l'optimiseur choisit de ne pas créer un index temporaire.
La requête que j'utilise est
SELECT
z.a
FROM dbo.t5 AS z WITH(INDEX(0))
WHERE
EXISTS
(
SELECT y.a
FROM dbo.t4 AS y
WHERE y.a = z.a
)
OPTION (MAXDOP 1);
Les schémas de table sont:
CREATE TABLE dbo.t4
(
a integer NULL,
b varchar(1000) NULL,
p varchar(100) NULL
);
CREATE TABLE dbo.t5
(
a integer NULL,
b varchar(1000) NULL
);
CREATE UNIQUE CLUSTERED INDEX c1
ON dbo.t5 (a);
Les deux tables ont 10 000 lignes, avec lesquelles vous pouvez simuler:
UPDATE STATISTICS dbo.t4
WITH
ROWCOUNT = 10000,
PAGECOUNT = 1000;
UPDATE STATISTICS dbo.t5
WITH
ROWCOUNT = 10000,
PAGECOUNT = 1000;
Le plan de requête est:
<?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="1.2" Build="11.0.2218.0" xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan">
<BatchSequence>
<Batch>
<Statements>
<StmtSimple StatementCompId="1" StatementEstRows="5532.16" StatementId="1" StatementOptmLevel="FULL" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" StatementSubTreeCost="0.407384" StatementText="select a from t5 z WITH(INDEX(0)) where exists (select a from t4 where a=z.a )" StatementType="SELECT" QueryHash="0x1B882FCEA34AEAF4" QueryPlanHash="0x1B276DC04B718F7C" RetrievedFromCache="true">
<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="2912" CachedPlanSize="32" CompileTime="10" CompileCPU="10" CompileMemory="296">
<MissingIndexes>
<MissingIndexGroup Impact="82.4536">
<MissingIndex Database="[planoper]" Schema="[dbo]" Table="[t4]">
<ColumnGroup Usage="EQUALITY">
<Column Name="[a]" ColumnId="1" />
</ColumnGroup>
</MissingIndex>
</MissingIndexGroup>
</MissingIndexes>
<MemoryGrantInfo SerialRequiredMemory="1024" SerialDesiredMemory="2912" RequiredMemory="1024" DesiredMemory="2912" RequestedMemory="2912" GrantWaitTime="0" GrantedMemory="2912" MaxUsedMemory="896" />
<OptimizerHardwareDependentProperties EstimatedAvailableMemoryGrant="104846" EstimatedPagesCached="11834" EstimatedAvailableDegreeOfParallelism="2" />
<RelOp AvgRowSize="11" EstimateCPU="0.228447" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="5532.16" LogicalOp="Left Semi Join" NodeId="0" Parallel="false" PhysicalOp="Hash Match" EstimatedTotalSubtreeCost="0.407384">
<OutputList>
<ColumnReference Database="[planoper]" Schema="[dbo]" Table="[t5]" Alias="[z]" Column="a" />
</OutputList>
<MemoryFractions Input="1" Output="1" />
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="10000" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<Hash>
<DefinedValues />
<HashKeysBuild>
<ColumnReference Database="[planoper]" Schema="[dbo]" Table="[t5]" Alias="[z]" Column="a" />
</HashKeysBuild>
<HashKeysProbe>
<ColumnReference Database="[planoper]" Schema="[dbo]" Table="[t4]" Column="a" />
</HashKeysProbe>
<ProbeResidual>
<ScalarOperator ScalarString="[planoper].[dbo].[t4].[a]=[planoper].[dbo].[t5].[a] as [z].[a]">
<Compare CompareOp="EQ">
<ScalarOperator>
<Identifier>
<ColumnReference Database="[planoper]" Schema="[dbo]" Table="[t4]" Column="a" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Identifier>
<ColumnReference Database="[planoper]" Schema="[dbo]" Table="[t5]" Alias="[z]" Column="a" />
</Identifier>
</ScalarOperator>
</Compare>
</ScalarOperator>
</ProbeResidual>
<RelOp AvgRowSize="11" EstimateCPU="0.0110785" EstimateIO="0.0565368" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="10000" LogicalOp="Clustered Index Scan" NodeId="1" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0676153" TableCardinality="10000">
<OutputList>
<ColumnReference Database="[planoper]" Schema="[dbo]" Table="[t5]" Alias="[z]" Column="a" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="10000" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<IndexScan Ordered="false" ForcedIndex="true" ForceScan="false" NoExpandHint="false">
<DefinedValues>
<DefinedValue>
<ColumnReference Database="[planoper]" Schema="[dbo]" Table="[t5]" Alias="[z]" Column="a" />
</DefinedValue>
</DefinedValues>
<Object Database="[planoper]" Schema="[dbo]" Table="[t5]" Index="[c1]" Alias="[z]" IndexKind="Clustered" />
</IndexScan>
</RelOp>
<RelOp AvgRowSize="11" EstimateCPU="0.011157" EstimateIO="0.100162" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="10000" LogicalOp="Table Scan" NodeId="2" Parallel="false" PhysicalOp="Table Scan" EstimatedTotalSubtreeCost="0.111319" TableCardinality="10000">
<OutputList>
<ColumnReference Database="[planoper]" Schema="[dbo]" Table="[t4]" Column="a" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="10000" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<TableScan Ordered="false" ForcedIndex="false" ForceScan="false" NoExpandHint="false">
<DefinedValues>
<DefinedValue>
<ColumnReference Database="[planoper]" Schema="[dbo]" Table="[t4]" Column="a" />
</DefinedValue>
</DefinedValues>
<Object Database="[planoper]" Schema="[dbo]" Table="[t4]" IndexKind="Heap" />
</TableScan>
</RelOp>
</Hash>
</RelOp>
</QueryPlan>
</StmtSimple>
</Statements>
</Batch>
</BatchSequence>
</ShowPlanXML>
Il me dit même de créer cet index:
USE [planoper];
GO
CREATE NONCLUSTERED INDEX [<Name of Missing Index, sysname,>]
ON [dbo].[t4] ([a]);
Pas de solution correcte
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