Question

我有一个包含450,000行的表，其中varchar列的长度可变（6到13个字符之间，分布不均匀）。我需要使用标准来加入另一个表，该标准表示目标表中的列以第一个表的列的值开头。

在我目前的测试样本中，我知道所有匹配都是6个字符，所以我正在使用t1.Digits = left(t2.Number, 6)进行连接，这非常快（运行我的大查询几秒钟）。我的测试样本是10,000条记录，但在生产中，查询需要运行数十万条。

我也知道绝大多数记录总是6个字符的匹配，但我需要支持更多的匹配，否则我有时会得到重复的记录。问题是我已经尝试了以下所有这些方法，并且每个方法都比左边六个字符的简单连接慢得多。我从来没有让他们跑超过五分钟，但他们没有表现出任何终止的迹象：

t1.Digits = left(t2.Number, datalength(t1.Digits))
charindex(t1.Digits, t2.Number) = 1
将预先计算的DigitLength int列添加到t1，然后使用t1.Digits = left(t2.Number, t1.DigitLength)
t2.Number like t1.Digits + '%'

以上四种解决方案中的每一种都在理论上实现了我想要的，但对于我的目的来说运行速度太慢。

即使这些列中的值是数字，我也使用varchar，因为在许多情况下需要保留前导零。在任何情况下，即使数据是字母数字的情况，也应该有一个快速的解决方案。

是否有人意识到一种非常快速的“开始”逻辑，其性能可与我过于简单的连接相媲美？

我在t1.Digits列上有聚集索引吗？

以下是使用上述方法＃4运行的执行计划：

    <?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="10720" StatementId="1" StatementOptmLevel="FULL" StatementSubTreeCost="7471.7" StatementText="select c.FromNumber, c.ToNumber, d.Destination, d.Digits&#xD;&#xA;from Converting c&#xD;&#xA;--join CASH.CASH.dbo.DestinationLookup d on d.Digits = left(c.FromNumber, 6) &#xD;&#xA;join CASH.CASH.dbo.DestinationLookup d on c.FromNumber like d.Digits + '%' &#xD;&#xA;" 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="114" CachedPlanSize="99" CompileTime="36" CompileCPU="35" CompileMemory="312">
            <RelOp AvgRowSize="77" EstimateCPU="174.861" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="10720" LogicalOp="Inner Join" NodeId="0" Parallel="false" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="7471.7">
              <OutputList>
                <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="ToNumber" />
                <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
              </OutputList>
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRows="10720" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <NestedLoops Optimized="false">
                <OuterReferences>
                  <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                </OuterReferences>
                <RelOp AvgRowSize="38" EstimateCPU="0.164714" EstimateIO="0.00281532" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="10720" LogicalOp="Sort" NodeId="1" Parallel="false" PhysicalOp="Sort" EstimatedTotalSubtreeCost="0.340338">
                  <OutputList>
                    <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                    <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="ToNumber" />
                  </OutputList>
                  <MemoryFractions Input="1" Output="1" />
                  <RunTimeInformation>
                    <RunTimeCountersPerThread Thread="0" ActualRebinds="1" ActualRewinds="0" ActualRows="10720" ActualEndOfScans="1" ActualExecutions="1" />
                  </RunTimeInformation>
                  <Sort Distinct="false">
                    <OrderBy>
                      <OrderByColumn Ascending="true">
                        <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                      </OrderByColumn>
                    </OrderBy>
                    <RelOp AvgRowSize="38" EstimateCPU="0.00296763" EstimateIO="0.126907" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="10720" LogicalOp="Table Scan" NodeId="2" Parallel="false" PhysicalOp="Table Scan" EstimatedTotalSubtreeCost="0.129875">
                      <OutputList>
                        <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                        <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="ToNumber" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="0" ActualRows="10720" ActualEndOfScans="1" ActualExecutions="1" />
                      </RunTimeInformation>
                      <TableScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                        <DefinedValues>
                          <DefinedValue>
                            <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="ToNumber" />
                          </DefinedValue>
                        </DefinedValues>
                        <Object Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" />
                      </TableScan>
                    </RelOp>
                  </Sort>
                </RelOp>
                <RelOp AvgRowSize="48" EstimateCPU="0.00290986" EstimateIO="0.01" EstimateRebinds="1390" EstimateRewinds="9329" EstimateRows="15609.2" LogicalOp="Lazy Spool" NodeId="3" Parallel="false" PhysicalOp="Table Spool" EstimatedTotalSubtreeCost="7296.5">
                  <OutputList>
                    <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                    <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                  </OutputList>
                  <RunTimeInformation>
                    <RunTimeCountersPerThread Thread="0" ActualRebinds="1391" ActualRewinds="9329" ActualRows="10720" ActualEndOfScans="10720" ActualExecutions="10720" />
                  </RunTimeInformation>
                  <Spool>
                    <RelOp AvgRowSize="48" EstimateCPU="5.21308" EstimateIO="0" EstimateRebinds="1390" EstimateRewinds="0" EstimateRows="15609.2" LogicalOp="Compute Scalar" NodeId="4" Parallel="false" PhysicalOp="Compute Scalar" EstimatedTotalSubtreeCost="7251.4">
                      <OutputList>
                        <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                        <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                      </OutputList>
                      <ComputeScalar>
                        <DefinedValues>
                          <DefinedValue>
                            <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                            <ScalarOperator ScalarString="[CASH].[CASH].[dbo].[DestinationLookup].[Digits] as [d].[Digits]">
                              <Identifier>
                                <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                              </Identifier>
                            </ScalarOperator>
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                            <ScalarOperator ScalarString="[CASH].[CASH].[dbo].[DestinationLookup].[Destination] as [d].[Destination]">
                              <Identifier>
                                <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                              </Identifier>
                            </ScalarOperator>
                          </DefinedValue>
                        </DefinedValues>
                        <RelOp AvgRowSize="48" EstimateCPU="5.21308" EstimateIO="0" EstimateRebinds="1390" EstimateRewinds="0" EstimateRows="15609.2" LogicalOp="Remote Query" NodeId="5" Parallel="false" PhysicalOp="Remote Query" EstimatedTotalSubtreeCost="7251.4">
                          <OutputList>
                            <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                            <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="0" ActualRebinds="1391" ActualRewinds="0" ActualRows="1391" ActualEndOfScans="1391" ActualExecutions="1391" />
                          </RunTimeInformation>
                          <RemoteQuery RemoteSource="CASH" RemoteQuery="SELECT &quot;Tbl1004&quot;.&quot;Digits&quot; &quot;Col1021&quot;,&quot;Tbl1004&quot;.&quot;Destination&quot; &quot;Col1022&quot; FROM &quot;CASH&quot;.&quot;dbo&quot;.&quot;DestinationLookup&quot; &quot;Tbl1004&quot; WHERE ? like &quot;Tbl1004&quot;.&quot;Digits&quot;+'%'" />
                        </RelOp>
                      </ComputeScalar>
                    </RelOp>
                  </Spool>
                </RelOp>
              </NestedLoops>
            </RelOp>
          </QueryPlan>
        </StmtSimple>
      </Statements>
    </Batch>
  </BatchSequence>
</ShowPlanXML>

这是加入简单左边的计划（t2.Number，6）：

    <?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="10720" StatementId="1" StatementOptmLevel="FULL" StatementSubTreeCost="15.1845" StatementText="select c.FromNumber, c.ToNumber, d.Destination, d.Digits&#xD;&#xA;from Converting c&#xD;&#xA;join CASH.CASH.dbo.DestinationLookup d on d.Digits = left(c.FromNumber, 6) " 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" CachedPlanSize="105" CompileTime="60" CompileCPU="58" CompileMemory="360">
            <RelOp AvgRowSize="77" EstimateCPU="0.0448096" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="10720" LogicalOp="Inner Join" NodeId="0" Parallel="false" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="15.1845">
              <OutputList>
                <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="ToNumber" />
                <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
              </OutputList>
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRows="10720" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <NestedLoops Optimized="false">
                <OuterReferences>
                  <ColumnReference Column="Expr1005" />
                </OuterReferences>
                <RelOp AvgRowSize="43" EstimateCPU="0.001072" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="10720" LogicalOp="Compute Scalar" NodeId="1" Parallel="false" PhysicalOp="Compute Scalar" EstimatedTotalSubtreeCost="0.13985">
                  <OutputList>
                    <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                    <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="ToNumber" />
                    <ColumnReference Column="Expr1005" />
                  </OutputList>
                  <ComputeScalar>
                    <DefinedValues>
                      <DefinedValue>
                        <ColumnReference Column="Expr1005" />
                        <ScalarOperator ScalarString="substring([CASH].[dbo].[Converting].[FromNumber] as [c].[FromNumber],(1),(6))">
                          <Intrinsic FunctionName="substring">
                            <ScalarOperator>
                              <Identifier>
                                <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                              </Identifier>
                            </ScalarOperator>
                            <ScalarOperator>
                              <Const ConstValue="(1)" />
                            </ScalarOperator>
                            <ScalarOperator>
                              <Const ConstValue="(6)" />
                            </ScalarOperator>
                          </Intrinsic>
                        </ScalarOperator>
                      </DefinedValue>
                    </DefinedValues>
                    <RelOp AvgRowSize="38" EstimateCPU="0.011949" EstimateIO="0.126829" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="10720" LogicalOp="Table Scan" NodeId="2" Parallel="false" PhysicalOp="Table Scan" EstimatedTotalSubtreeCost="0.138778">
                      <OutputList>
                        <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                        <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="ToNumber" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="0" ActualRows="10720" ActualEndOfScans="1" ActualExecutions="1" />
                      </RunTimeInformation>
                      <TableScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                        <DefinedValues>
                          <DefinedValue>
                            <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="FromNumber" />
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" Column="ToNumber" />
                          </DefinedValue>
                        </DefinedValues>
                        <Object Database="[CASH]" Schema="[dbo]" Table="[Converting]" Alias="[c]" />
                      </TableScan>
                    </RelOp>
                  </ComputeScalar>
                </RelOp>
                <RelOp AvgRowSize="48" EstimateCPU="0.000258212" EstimateIO="0.003125" EstimateRebinds="10580.9" EstimateRewinds="138.124" EstimateRows="1" LogicalOp="Lazy Spool" NodeId="6" Parallel="false" PhysicalOp="Index Spool" EstimatedTotalSubtreeCost="14.9998">
                  <OutputList>
                    <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                    <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                  </OutputList>
                  <RunTimeInformation>
                    <RunTimeCountersPerThread Thread="0" ActualRebinds="830" ActualRewinds="9890" ActualRows="10720" ActualEndOfScans="0" ActualExecutions="10720" />
                  </RunTimeInformation>
                  <Spool>
                    <SeekPredicate>
                      <Prefix ScanType="EQ">
                        <RangeColumns>
                          <ColumnReference Column="Expr1005" />
                        </RangeColumns>
                        <RangeExpressions>
                          <ScalarOperator ScalarString="[Expr1005]">
                            <Identifier>
                              <ColumnReference Column="Expr1005" />
                            </Identifier>
                          </ScalarOperator>
                        </RangeExpressions>
                      </Prefix>
                    </SeekPredicate>
                    <RelOp AvgRowSize="48" EstimateCPU="0.0103333" EstimateIO="0" EstimateRebinds="1180" EstimateRewinds="0" EstimateRows="1" LogicalOp="Compute Scalar" NodeId="7" Parallel="false" PhysicalOp="Compute Scalar" EstimatedTotalSubtreeCost="12.2037">
                      <OutputList>
                        <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                        <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                      </OutputList>
                      <ComputeScalar>
                        <DefinedValues>
                          <DefinedValue>
                            <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                            <ScalarOperator ScalarString="[CASH].[CASH].[dbo].[DestinationLookup].[Digits] as [d].[Digits]">
                              <Identifier>
                                <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                              </Identifier>
                            </ScalarOperator>
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                            <ScalarOperator ScalarString="[CASH].[CASH].[dbo].[DestinationLookup].[Destination] as [d].[Destination]">
                              <Identifier>
                                <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                              </Identifier>
                            </ScalarOperator>
                          </DefinedValue>
                        </DefinedValues>
                        <RelOp AvgRowSize="48" EstimateCPU="0.0103333" EstimateIO="0" EstimateRebinds="1180" EstimateRewinds="0" EstimateRows="1" LogicalOp="Remote Query" NodeId="8" Parallel="false" PhysicalOp="Remote Query" EstimatedTotalSubtreeCost="12.2037">
                          <OutputList>
                            <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Digits" />
                            <ColumnReference Server="[CASH]" Database="[CASH]" Schema="[dbo]" Table="[DestinationLookup]" Alias="[d]" Column="Destination" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="0" ActualRebinds="456" ActualRewinds="0" ActualRows="456" ActualEndOfScans="0" ActualExecutions="456" />
                          </RunTimeInformation>
                          <RemoteQuery RemoteSource="CASH" RemoteQuery="SELECT &quot;Tbl1004&quot;.&quot;Digits&quot; &quot;Col1015&quot;,&quot;Tbl1004&quot;.&quot;Destination&quot; &quot;Col1016&quot; FROM &quot;CASH&quot;.&quot;dbo&quot;.&quot;DestinationLookup&quot; &quot;Tbl1004&quot; WHERE &quot;Tbl1004&quot;.&quot;Digits&quot;=?" />
                        </RelOp>
                      </ComputeScalar>
                    </RelOp>
                  </Spool>
                </RelOp>
              </NestedLoops>
            </RelOp>
          </QueryPlan>
        </StmtSimple>
      </Statements>
    </Batch>
  </BatchSequence>
</ShowPlanXML>

更新：我一直无法找到理想的解决方案，但我发现了下一个最好的解决方案。似乎使用“like”对这两个表进行简单的简单查询在大约五秒钟内完成。因此，我没有尝试将连接填充到我的怪物查询中，它从未完成，我用它来创建一个临时查找表，然后我的怪物查询使用它。总之，大查询现在在9秒内完成，我在varchar join中支持可变长度字符串。

另一个有助于提高速度的方法是将t1中列的填充因子从80更改为100.此填充因子非常适合表格，因为它是一个静态参考表，每年只更改一次。

Answer 1

这四个中性能最佳的解决方案是第四个。

让我们设置测试环境：

create table #t1 (digits varchar(10), filler char(5000) default(''))
create table #t2 (number varchar(10), filler char(5000) default(''))
go

insert #t1 (digits) values
('123'),('234'),('345'),('456'),('567')

insert #t2 (number) values
('1234'),('234'),('345689'),('45'),('567890')
go

create index ix_t2 on #t2(number);
go

现在，让我们执行四个语义相同的查询，但使用Query - ＆gt;包括启用的实际执行计划，以及SET STATISTICS IO ON：

-- 1
select *
from #t1
inner join #t2
on #t1.digits = left(#t2.number, datalength(#t1.digits))

-- 2
select *
from #t1
inner join #t2
on charindex(#t1.Digits, #t2.Number) = 1

-- 3
select *
from #t1
inner join #t2
on charindex(#t1.digits, #t2.number) = 1

-- 4
select *
from #t1
inner join #t2
on #t2.number like #t1.digits + '%'

如您所见，执行计划为1,2和＆amp; 3包括两个表上的表扫描运算符（包括第一个的附加计算标量运算符），但第四个查询对＃t2上的索引执行索引查找。此外，如果检查统计信息io的输出，您将看到对于1,2和＆amp;的＃t2（带索引的表）的逻辑读取度量。 3为25，但第四个只有14（当然，行数越多，数字就越高）。

Answer 2

在table1.digits上建立索引。然后尝试以下方法：

select t2.*, t1.<whatever>
from table1 t2 cross apply
     (select top 1 <whatever>
      from table1 t1
      where t1.digits <= t2.number
      order by t1.digits desc
     ) t1;

SQL Server有时比常规连接更优化“应用”查询。在这种情况下，它可能会发现索引对where和order by都有用，并且可以有效地进行。（我也认为同样适用于相关子查询。）

优化SQL Server中的“开始于”比较

2 个答案: