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I know its something that should be avoided for performance reasons, but am trying to show a condition where it appears as a demo on how to make sure it does not appear.

However, I end up with a missing index warning, yet the optimizer chooses not to create a temporary index.

The query I am using is

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);

Table schemas are:

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);

Both tables have 10,000 rows, which you can simulate with:

UPDATE STATISTICS dbo.t4 
WITH 
    ROWCOUNT = 10000, 
    PAGECOUNT = 1000;

UPDATE STATISTICS dbo.t5 
WITH 
    ROWCOUNT = 10000,
    PAGECOUNT = 1000;

The query plan is:

default plan

<?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>

It even tells me to create this index:

USE [planoper];
GO
CREATE NONCLUSTERED INDEX [<Name of Missing Index, sysname,>]
ON [dbo].[t4] ([a]);
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2  
You may want to take a look at this simple-talk.com/sql/learn-sql-server/… –  Zane Dec 11 '12 at 15:46
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1 Answer

up vote 17 down vote accepted

One way to get an index spool to appear naturally is to express the requirement using slightly different syntax:

SELECT DISTINCT 
    z.a
FROM dbo.t5 AS z
JOIN dbo.t4 AS y ON
    y.a >= z.a AND y.a <= z.a
OPTION (LOOP JOIN, MAXDOP 1, FORCE ORDER);

This produces an execution plan like:

Join inequality plan

Rewriting the equality as a pair of equivalent inequalities encourages the use of an index spool, though the spooled predicate is not exactly what you were after, the semantics are ultimately the same.

There is no easy way to introduce the desired index spool using the original syntax; however, that's not to say it is impossible. Since you only need this for a demo, and will not be using this anywhere near a production system, I will show you another way:

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, LOOP JOIN, QUERYTRACEON 9114);

The execution plan is:

Trace flag plan

The index spool predicate is as desired:

Seek Keys[1]: Prefix: [dbo].[t4].a = [dbo].[t5].[a] as [z].[a]

You will not be able to use this plan in a USE PLAN hint because the optimizer would not normally consider it; nor will I be saying very much about how it works, except to say that QUERYTRACEON turns an undocumented trace flag on for this single statement only.

share|improve this answer
    
Tried the 1st method, and it works. Thanks. But I get a row count spool after the index spool which you do not get in the plan you have shown. Any idea why? –  Akash Dec 12 '12 at 4:33
    
Updated the statistics, though I have the actual data in the table. Still getting the row count spool, guess its a build difference since I'm on 11.0.2218 –  Akash Dec 12 '12 at 5:23
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