Question

我们目前正试图加速一些查询，我们遇到了一些我（不是DBA只是.NET开发人员）无法解释或理解的东西。我们在SQL Server 2005上运行此查询。

我们有以下查询（为了参数而做的小而简单）;

SELECT    
    *
FROM    
    RandomTable 
WHERE   
    MoneyColumn <> 0
GROUP BY
    SomeColumn

这个查询在三秒左右运行，然后我们随机尝试跟随加速（真的在黑暗中拍摄）

SELECT    
    *
FROM    
    RandomTable 
WHERE   
    isnull(MoneyColumn,0) <> 0
GROUP BY
    SomeColumn

这会将查询速度降低到一秒......

这个列没有NULL值（但是由于数据库设计很糟糕）但是它是空的......

事实是它是无效的，这使得SQL Server可以做些什么来解决这个问题，这使得ISNULL没有被提到它的速度很慢？我根本不知道为什么ISNULL会让它表现得更快（并且如此大的余地）。我认为当查询中有ISNULL语句时，SQL实际上还有更多工作要做。

任何人都可以对此有所了解吗？

编辑已添加执行计划

使用ISNULL

<?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="1.0" Build="9.00.5000.00" xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan">
  <BatchSequence>
    <Batch>
      <Statements>
        <StmtSimple StatementCompId="1" StatementEstRows="9019.76" StatementId="1" StatementOptmLevel="FULL" StatementSubTreeCost="1.48105" StatementText="SELECT    debiteur_id, MIN(Faktuurdatum) AS OldestOpenInvoiceDate, ISNULL(SUM(Totaal_Open),0) AS TotalOpenAmount&#xD;&#xA;FROM    dbo.tbl_Faktuur &#xD;&#xA;WHERE   (Afgehandeld_NeeJa = 0 OR Afgehandeld_NeeJa IS NULL)&#xD;&#xA;AND        (ISNULL(Totaal_Open,0) &lt;&gt; 0) &#xD;&#xA;--AND        (Totaal_Open &lt;&gt; 0) &#xD;&#xA;GROUP BY debiteur_id" StatementType="SELECT">
          <StatementSetOptions ANSI_NULLS="false" ANSI_PADDING="false" ANSI_WARNINGS="false" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="false" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="false" />
          <QueryPlan DegreeOfParallelism="1" MemoryGrant="1520" CachedPlanSize="54" CompileTime="11" CompileCPU="11" CompileMemory="704">
            <RelOp AvgRowSize="23" EstimateCPU="0.000901976" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="9019.76" LogicalOp="Compute Scalar" NodeId="0" Parallel="false" PhysicalOp="Compute Scalar" EstimatedTotalSubtreeCost="1.48105">
              <OutputList>
                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                <ColumnReference Column="Expr1003" />
                <ColumnReference Column="Expr1005" />
              </OutputList>
              <ComputeScalar>
                <DefinedValues>
                  <DefinedValue>
                    <ColumnReference Column="Expr1005" />
                    <ScalarOperator ScalarString="isnull([Expr1004],($0.0000))">
                      <Intrinsic FunctionName="isnull">
                        <ScalarOperator>
                          <Identifier>
                            <ColumnReference Column="Expr1004" />
                          </Identifier>
                        </ScalarOperator>
                        <ScalarOperator>
                          <Const ConstValue="($0.0000)" />
                        </ScalarOperator>
                      </Intrinsic>
                    </ScalarOperator>
                  </DefinedValue>
                </DefinedValues>
                <RelOp AvgRowSize="23" EstimateCPU="0.291662" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="9019.76" LogicalOp="Compute Scalar" NodeId="1" Parallel="false" PhysicalOp="Compute Scalar" EstimatedTotalSubtreeCost="1.48014">
                  <OutputList>
                    <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                    <ColumnReference Column="Expr1003" />
                    <ColumnReference Column="Expr1004" />
                  </OutputList>
                  <ComputeScalar>
                    <DefinedValues>
                      <DefinedValue>
                        <ColumnReference Column="Expr1004" />
                        <ScalarOperator ScalarString="CASE WHEN [Expr1013]=(0) THEN NULL ELSE [Expr1014] END">
                          <IF>
                            <Condition>
                              <ScalarOperator>
                                <Compare CompareOp="EQ">
                                  <ScalarOperator>
                                    <Identifier>
                                      <ColumnReference Column="Expr1013" />
                                    </Identifier>
                                  </ScalarOperator>
                                  <ScalarOperator>
                                    <Const ConstValue="(0)" />
                                  </ScalarOperator>
                                </Compare>
                              </ScalarOperator>
                            </Condition>
                            <Then>
                              <ScalarOperator>
                                <Const ConstValue="NULL" />
                              </ScalarOperator>
                            </Then>
                            <Else>
                              <ScalarOperator>
                                <Identifier>
                                  <ColumnReference Column="Expr1014" />
                                </Identifier>
                              </ScalarOperator>
                            </Else>
                          </IF>
                        </ScalarOperator>
                      </DefinedValue>
                    </DefinedValues>
                    <RelOp AvgRowSize="23" EstimateCPU="0.291662" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="9019.76" LogicalOp="Aggregate" NodeId="2" Parallel="false" PhysicalOp="Hash Match" EstimatedTotalSubtreeCost="1.48014">
                      <OutputList>
                        <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                        <ColumnReference Column="Expr1003" />
                        <ColumnReference Column="Expr1013" />
                        <ColumnReference Column="Expr1014" />
                      </OutputList>
                      <MemoryFractions Input="1" Output="1" />
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="0" ActualRows="156794" ActualEndOfScans="1" ActualExecutions="1" />
                      </RunTimeInformation>
                      <Hash>
                        <DefinedValues>
                          <DefinedValue>
                            <ColumnReference Column="Expr1003" />
                            <ScalarOperator ScalarString="MIN([directpay].[dbo].[tbl_Faktuur].[Faktuurdatum])">
                              <Aggregate AggType="MIN" Distinct="false">
                                <ScalarOperator>
                                  <Identifier>
                                    <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Faktuurdatum" />
                                  </Identifier>
                                </ScalarOperator>
                              </Aggregate>
                            </ScalarOperator>
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Column="Expr1013" />
                            <ScalarOperator ScalarString="COUNT_BIG([directpay].[dbo].[tbl_Faktuur].[Totaal_Open])">
                              <Aggregate AggType="COUNT_BIG" Distinct="false">
                                <ScalarOperator>
                                  <Identifier>
                                    <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                                  </Identifier>
                                </ScalarOperator>
                              </Aggregate>
                            </ScalarOperator>
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Column="Expr1014" />
                            <ScalarOperator ScalarString="SUM([directpay].[dbo].[tbl_Faktuur].[Totaal_Open])">
                              <Aggregate AggType="SUM" Distinct="false">
                                <ScalarOperator>
                                  <Identifier>
                                    <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                                  </Identifier>
                                </ScalarOperator>
                              </Aggregate>
                            </ScalarOperator>
                          </DefinedValue>
                        </DefinedValues>
                        <HashKeysBuild>
                          <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                        </HashKeysBuild>
                        <RelOp AvgRowSize="23" EstimateCPU="0.255" EstimateIO="0.634196" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="27420" LogicalOp="Index Seek" NodeId="4" Parallel="false" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.889196">
                          <OutputList>
                            <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                            <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Faktuurdatum" />
                            <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="0" ActualRows="298726" ActualEndOfScans="1" ActualExecutions="1" />
                          </RunTimeInformation>
                          <IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" NoExpandHint="false">
                            <DefinedValues>
                              <DefinedValue>
                                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                              </DefinedValue>
                              <DefinedValue>
                                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Faktuurdatum" />
                              </DefinedValue>
                              <DefinedValue>
                                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                              </DefinedValue>
                            </DefinedValues>
                            <Object Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Index="[_dta_index_tbl_Faktuur_5_583009158__K13_K9_K19_K2_5]" />
                            <SeekPredicates>
                              <SeekPredicate>
                                <Prefix ScanType="EQ">
                                  <RangeColumns>
                                    <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Afgehandeld_NeeJa" />
                                  </RangeColumns>
                                  <RangeExpressions>
                                    <ScalarOperator ScalarString="(0)">
                                      <Const ConstValue="(0)" />
                                    </ScalarOperator>
                                  </RangeExpressions>
                                </Prefix>
                              </SeekPredicate>
                            </SeekPredicates>
                            <Predicate>
                              <ScalarOperator ScalarString="isnull([directpay].[dbo].[tbl_Faktuur].[Totaal_Open],($0.0000))&lt;($0.0000) OR isnull([directpay].[dbo].[tbl_Faktuur].[Totaal_Open],($0.0000))&gt;($0.0000)">
                                <Logical Operation="OR">
                                  <ScalarOperator>
                                    <Compare CompareOp="LT">
                                      <ScalarOperator>
                                        <Intrinsic FunctionName="isnull">
                                          <ScalarOperator>
                                            <Identifier>
                                              <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                                            </Identifier>
                                          </ScalarOperator>
                                          <ScalarOperator>
                                            <Const ConstValue="($0.0000)" />
                                          </ScalarOperator>
                                        </Intrinsic>
                                      </ScalarOperator>
                                      <ScalarOperator>
                                        <Const ConstValue="($0.0000)" />
                                      </ScalarOperator>
                                    </Compare>
                                  </ScalarOperator>
                                  <ScalarOperator>
                                    <Compare CompareOp="GT">
                                      <ScalarOperator>
                                        <Intrinsic FunctionName="isnull">
                                          <ScalarOperator>
                                            <Identifier>
                                              <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                                            </Identifier>
                                          </ScalarOperator>
                                          <ScalarOperator>
                                            <Const ConstValue="($0.0000)" />
                                          </ScalarOperator>
                                        </Intrinsic>
                                      </ScalarOperator>
                                      <ScalarOperator>
                                        <Const ConstValue="($0.0000)" />
                                      </ScalarOperator>
                                    </Compare>
                                  </ScalarOperator>
                                </Logical>
                              </ScalarOperator>
                            </Predicate>
                          </IndexScan>
                        </RelOp>
                      </Hash>
                    </RelOp>
                  </ComputeScalar>
                </RelOp>
              </ComputeScalar>
            </RelOp>
          </QueryPlan>
        </StmtSimple>
      </Statements>
    </Batch>
  </BatchSequence>
</ShowPlanXML>

没有ISNULL

<?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="1.0" Build="9.00.5000.00" xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan">
  <BatchSequence>
    <Batch>
      <Statements>
        <StmtSimple StatementCompId="1" StatementEstRows="1322.43" StatementId="1" StatementOptmLevel="FULL" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" StatementSubTreeCost="0.274954" StatementText="SELECT    debiteur_id, MIN(Faktuurdatum) AS OldestOpenInvoiceDate, ISNULL(SUM(Totaal_Open),0) AS TotalOpenAmount&#xD;&#xA;FROM    dbo.tbl_Faktuur &#xD;&#xA;WHERE   (Afgehandeld_NeeJa = 0 OR Afgehandeld_NeeJa IS NULL)&#xD;&#xA;--AND        (ISNULL(Totaal_Open,0) &lt;&gt; 0) &#xD;&#xA;AND        (Totaal_Open &lt;&gt; 0) &#xD;&#xA;GROUP BY debiteur_id" StatementType="SELECT">
          <StatementSetOptions ANSI_NULLS="false" ANSI_PADDING="false" ANSI_WARNINGS="false" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="false" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="false" />
          <QueryPlan CachedPlanSize="47" CompileTime="9" CompileCPU="9" CompileMemory="528">
            <RelOp AvgRowSize="23" EstimateCPU="0.000132243" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1322.43" LogicalOp="Compute Scalar" NodeId="0" Parallel="false" PhysicalOp="Compute Scalar" EstimatedTotalSubtreeCost="0.274954">
              <OutputList>
                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                <ColumnReference Column="Expr1003" />
                <ColumnReference Column="Expr1005" />
              </OutputList>
              <ComputeScalar>
                <DefinedValues>
                  <DefinedValue>
                    <ColumnReference Column="Expr1005" />
                    <ScalarOperator ScalarString="isnull([Expr1004],($0.0000))">
                      <Intrinsic FunctionName="isnull">
                        <ScalarOperator>
                          <Identifier>
                            <ColumnReference Column="Expr1004" />
                          </Identifier>
                        </ScalarOperator>
                        <ScalarOperator>
                          <Const ConstValue="($0.0000)" />
                        </ScalarOperator>
                      </Intrinsic>
                    </ScalarOperator>
                  </DefinedValue>
                </DefinedValues>
                <RelOp AvgRowSize="23" EstimateCPU="0.167304" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1322.43" LogicalOp="Aggregate" NodeId="1" Parallel="false" PhysicalOp="Hash Match" EstimatedTotalSubtreeCost="0.274822">
                  <OutputList>
                    <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                    <ColumnReference Column="Expr1003" />
                    <ColumnReference Column="Expr1004" />
                  </OutputList>
                  <MemoryFractions Input="0" Output="0" />
                  <Hash>
                    <DefinedValues>
                      <DefinedValue>
                        <ColumnReference Column="Expr1003" />
                        <ScalarOperator ScalarString="MIN([directpay].[dbo].[tbl_Faktuur].[Faktuurdatum])">
                          <Aggregate AggType="MIN" Distinct="false">
                            <ScalarOperator>
                              <Identifier>
                                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Faktuurdatum" />
                              </Identifier>
                            </ScalarOperator>
                          </Aggregate>
                        </ScalarOperator>
                      </DefinedValue>
                      <DefinedValue>
                        <ColumnReference Column="Expr1004" />
                        <ScalarOperator ScalarString="SUM([directpay].[dbo].[tbl_Faktuur].[Totaal_Open])">
                          <Aggregate AggType="SUM" Distinct="false">
                            <ScalarOperator>
                              <Identifier>
                                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                              </Identifier>
                            </ScalarOperator>
                          </Aggregate>
                        </ScalarOperator>
                      </DefinedValue>
                    </DefinedValues>
                    <HashKeysBuild>
                      <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                    </HashKeysBuild>
                    <RelOp AvgRowSize="23" EstimateCPU="0.030319" EstimateIO="0.0771991" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="27420" LogicalOp="Index Seek" NodeId="2" Parallel="false" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.107518">
                      <OutputList>
                        <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                        <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Faktuurdatum" />
                        <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                      </OutputList>
                      <IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" NoExpandHint="false">
                        <DefinedValues>
                          <DefinedValue>
                            <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Debiteur_ID" />
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Faktuurdatum" />
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                          </DefinedValue>
                        </DefinedValues>
                        <Object Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Index="[_dta_index_tbl_Faktuur_5_583009158__K13_K9_K19_K2_5]" />
                        <SeekPredicates>
                          <SeekPredicate>
                            <Prefix ScanType="EQ">
                              <RangeColumns>
                                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Afgehandeld_NeeJa" />
                              </RangeColumns>
                              <RangeExpressions>
                                <ScalarOperator ScalarString="(0)">
                                  <Const ConstValue="(0)" />
                                </ScalarOperator>
                              </RangeExpressions>
                            </Prefix>
                            <EndRange ScanType="LT">
                              <RangeColumns>
                                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                              </RangeColumns>
                              <RangeExpressions>
                                <ScalarOperator ScalarString="($0.0000)">
                                  <Const ConstValue="($0.0000)" />
                                </ScalarOperator>
                              </RangeExpressions>
                            </EndRange>
                          </SeekPredicate>
                          <SeekPredicate>
                            <Prefix ScanType="EQ">
                              <RangeColumns>
                                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Afgehandeld_NeeJa" />
                              </RangeColumns>
                              <RangeExpressions>
                                <ScalarOperator ScalarString="(0)">
                                  <Const ConstValue="(0)" />
                                </ScalarOperator>
                              </RangeExpressions>
                            </Prefix>
                            <StartRange ScanType="GT">
                              <RangeColumns>
                                <ColumnReference Database="[directpay]" Schema="[dbo]" Table="[tbl_Faktuur]" Column="Totaal_Open" />
                              </RangeColumns>
                              <RangeExpressions>
                                <ScalarOperator ScalarString="($0.0000)">
                                  <Const ConstValue="($0.0000)" />
                                </ScalarOperator>
                              </RangeExpressions>
                            </StartRange>
                          </SeekPredicate>
                        </SeekPredicates>
                      </IndexScan>
                    </RelOp>
                  </Hash>
                </RelOp>
              </ComputeScalar>
            </RelOp>
          </QueryPlan>
        </StmtSimple>
      </Statements>
    </Batch>
  </BatchSequence>
</ShowPlanXML>

Answer 1

ISNULL()版本构成了谓词unsargable的一部分。

除了意味着它不能有效地使用你的索引（它仍然可以从查询中的另一个谓词中寻找前导列），这也使得构造在估计时对优化器更加不透明。匹配的行数。

两个计划都有大约27,420行进入哈希匹配运算符，但它们的估计行数有所不同。较快的计划估计为9,019，较慢的计划为1,322。

这两个估计实际上都是非常错误的，因为实际上您的组有156,794个不同的值。由于错误的估计，查询没有分配足够的内存来构建哈希表，并且两者都需要溢出到光盘。较慢的一个特别是记忆匮乏，所以这种情况多次发生。

您可以尝试更新现有统计信息或查看创建一些多列统计信息，以便尝试更准确地估计将返回的不同分组数量，从而分配不同的计划或更多内存

Answer 2

添加两个过滤的覆盖索引可能有所帮助（ISNULL为什么帮助提高速度的问题）：

USE Directpay

/*
    SELECT    debiteur_id, MIN(Faktuurdatum) AS OldestOpenInvoiceDate, ISNULL(SUM(Totaal_Open),0) AS TotalOpenAmount
    FROM    dbo.tbl_Faktuur 
    WHERE   (Afgehandeld_NeeJa = 0 OR Afgehandeld_NeeJa IS NULL)
    AND        (ISNULL(Totaal_Open,0) <> 0) 
    --AND        (Totaal_Open <> 0) 
    GROUP BY debiteur_id
*/

-- Prepare two filtered indices to split the OR condition
CREATE NONCLUSTERED INDEX IX_tbl_faktuur_debiteur_id_faktuurdatum_flt_1 ON tbl_Faktuur (debiteur_id, faktuurdatum) 
INCLUDE (Totaal_Open)
WHERE   (Afgehandeld_NeeJa = 0);

CREATE NONCLUSTERED INDEX IX_tbl_faktuur_debiteur_id_faktuurdatum_flt_2 ON tbl_Faktuur (debiteur_id, faktuurdatum) 
INCLUDE (Totaal_Open)
WHERE   (Afgehandeld_NeeJa IS NULL);


-- Rewrite the query
SELECT     debiteur_id
         , min(OldestOpenInvoiceDate)     AS OldestOpenInvoiceDate
         , isnull(sum(TotalOpenAmount),0) AS TotalOpenAmount
FROM
(
    SELECT debiteur_id
         , min(Faktuurdatum)           AS OldestOpenInvoiceDate
         , sum(Totaal_Open)            AS TotalOpenAmount
    FROM     dbo.tbl_Faktuur
    WHERE    Afgehandeld_NeeJa = 0
    GROUP BY debiteur_id

    UNION ALL 

    SELECT debiteur_id
         , min(Faktuurdatum)           AS OldestOpenInvoiceDate
         , sum(Totaal_Open)            AS TotalOpenAmount
    FROM     dbo.tbl_Faktuur
    WHERE    Afgehandeld_NeeJa IS NULL
    GROUP BY debiteur_id
) Unhandled
GROUP BY debiteur_id

为什么ISNULL会提高查询的性能？

2 个答案: