Question

我创建了以下索引来覆盖select top语句。

-- Column A, B have type of int
create unique index ix_ on T (A, B) with (data_compression = page) 
-- tried to create non-unique index too and the execution plan is the same

select top 20 A, B from T order by A, B -- 19 seconds
select top 20 A, B from T -- return result instantly

然而，它仍然需要一段时间（我的桌子上有19万秒，有5000万行），执行计划显示还有一个＆＃34;排序＆＃34;？

执行计划显示

选择（成本：0％）←顶部（成本：0％）←平行（聚集流）（成本：0％）←排序（前N排序）成本：93％←索引扫描（非集群）[T。 ix_]费用：7％

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="1.2" Build="12.0.4100.1" xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan">
  <BatchSequence>
    <Batch>
      <Statements>
        <StmtSimple StatementCompId="1" StatementEstRows="20" StatementId="1" StatementOptmLevel="FULL" CardinalityEstimationModelVersion="120" StatementSubTreeCost="552.009" StatementText="select  top 20 A A, B B --, checksum(*) cs&#xA;from T with (index(ix_))&#xA;order by A, B" StatementType="SELECT" QueryHash="0x1531573504856080" QueryPlanHash="0x5D4FED760C34AF43" 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="8" MemoryGrant="1024" CachedPlanSize="24" CompileTime="2" CompileCPU="2" CompileMemory="256">
            <ThreadStat Branches="1" UsedThreads="8">
              <ThreadReservation NodeId="0" ReservedThreads="8" />
            </ThreadStat>
            <MemoryGrantInfo SerialRequiredMemory="16" SerialDesiredMemory="24" RequiredMemory="896" DesiredMemory="960" RequestedMemory="1024" GrantWaitTime="0" GrantedMemory="1024" MaxUsedMemory="896" />
            <OptimizerHardwareDependentProperties EstimatedAvailableMemoryGrant="768000" EstimatedPagesCached="768000" EstimatedAvailableDegreeOfParallelism="8" />
            <RelOp AvgRowSize="15" EstimateCPU="2E-06" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="20" LogicalOp="Top" NodeId="0" Parallel="false" PhysicalOp="Top" EstimatedTotalSubtreeCost="552.009">
              <OutputList>
                <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="B" />
                <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="A" />
              </OutputList>
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRows="20" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <Top RowCount="false" IsPercent="false" WithTies="false">
                <TopExpression>
                  <ScalarOperator ScalarString="(20)">
                    <Const ConstValue="(20)" />
                  </ScalarOperator>
                </TopExpression>
                <RelOp AvgRowSize="15" EstimateCPU="0.0286101" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="20" LogicalOp="Gather Streams" NodeId="1" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="552.009">
                  <OutputList>
                    <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="B" />
                    <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="A" />
                  </OutputList>
                  <RunTimeInformation>
                    <RunTimeCountersPerThread Thread="0" ActualRows="20" ActualEndOfScans="0" ActualExecutions="1" />
                  </RunTimeInformation>
                  <Parallelism>
                    <OrderBy>
                      <OrderByColumn Ascending="true">
                        <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="A" />
                      </OrderByColumn>
                      <OrderByColumn Ascending="true">
                        <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="B" />
                      </OrderByColumn>
                    </OrderBy>
                    <RelOp AvgRowSize="15" EstimateCPU="212.739" EstimateIO="303.269" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="20" LogicalOp="TopN Sort" NodeId="2" Parallel="true" PhysicalOp="Sort" EstimatedTotalSubtreeCost="551.98">
                      <OutputList>
                        <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="B" />
                        <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="A" />
                      </OutputList>
                      <MemoryFractions Input="1" Output="1" />
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="8" ActualRebinds="1" ActualRewinds="0" ActualRows="12" Batches="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row" />
                        <RunTimeCountersPerThread Thread="3" ActualRebinds="1" ActualRewinds="0" ActualRows="12" Batches="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row" />
                        <RunTimeCountersPerThread Thread="7" ActualRebinds="1" ActualRewinds="0" ActualRows="12" Batches="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row" />
                        <RunTimeCountersPerThread Thread="6" ActualRebinds="1" ActualRewinds="0" ActualRows="20" Batches="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row" />
                        <RunTimeCountersPerThread Thread="5" ActualRebinds="1" ActualRewinds="0" ActualRows="12" Batches="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row" />
                        <RunTimeCountersPerThread Thread="4" ActualRebinds="1" ActualRewinds="0" ActualRows="12" Batches="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row" />
                        <RunTimeCountersPerThread Thread="2" ActualRebinds="1" ActualRewinds="0" ActualRows="12" Batches="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row" />
                        <RunTimeCountersPerThread Thread="1" ActualRebinds="1" ActualRewinds="0" ActualRows="12" Batches="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row" />
                        <RunTimeCountersPerThread Thread="0" ActualRebinds="0" ActualRewinds="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                      </RunTimeInformation>
                      <TopSort Distinct="false" Rows="20">
                        <OrderBy>
                          <OrderByColumn Ascending="true">
                            <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="A" />
                          </OrderByColumn>
                          <OrderByColumn Ascending="true">
                            <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="B" />
                          </OrderByColumn>
                        </OrderBy>
                        <RelOp AvgRowSize="15" EstimateCPU="5.81245" EstimateIO="30.16" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row" EstimateRows="42226500" LogicalOp="Index Scan" NodeId="3" Parallel="true" Partitioned="true" PhysicalOp="Index Scan" EstimatedTotalSubtreeCost="35.9724" TableCardinality="42226500">
                          <OutputList>
                            <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="B" />
                            <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="A" />
                          </OutputList>
                          <RunTimeInformation>
                            <RunTimeCountersPerThread Thread="8" ActualRows="3993270" Batches="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row" />
                            <RunTimeCountersPerThread Thread="7" ActualRows="2713924" Batches="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row" />
                            <RunTimeCountersPerThread Thread="6" ActualRows="8866373" Batches="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row" />
                            <RunTimeCountersPerThread Thread="5" ActualRows="10625143" Batches="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row" />
                            <RunTimeCountersPerThread Thread="4" ActualRows="4254726" Batches="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row" />
                            <RunTimeCountersPerThread Thread="3" ActualRows="3195887" Batches="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row" />
                            <RunTimeCountersPerThread Thread="2" ActualRows="3626671" Batches="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row" />
                            <RunTimeCountersPerThread Thread="1" ActualRows="4950538" Batches="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row" />
                            <RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
                          </RunTimeInformation>
                          <RunTimePartitionSummary>
                            <PartitionsAccessed PartitionCount="41">
                              <PartitionRange Start="1" End="41" />
                            </PartitionsAccessed>
                          </RunTimePartitionSummary>
                          <IndexScan Ordered="false" ForcedIndex="true" ForceSeek="false" ForceScan="false" NoExpandHint="false" Storage="RowStore">
                            <DefinedValues>
                              <DefinedValue>
                                <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="B" />
                              </DefinedValue>
                              <DefinedValue>
                                <ColumnReference Database="[DB]" Schema="[dbo]" Table="[T]" Alias="[T]" Column="A" />
                              </DefinedValue>
                            </DefinedValues>
                            <Object Database="[DB]" Schema="[dbo]" Table="[T]" Index="[ix_]" Alias="[T]" IndexKind="NonClustered" Storage="RowStore" />
                          </IndexScan>
                        </RelOp>
                      </TopSort>
                    </RelOp>
                  </Parallelism>
                </RelOp>
              </Top>
            </RelOp>
          </QueryPlan>
        </StmtSimple>
      </Statements>
    </Batch>
  </BatchSequence>
</ShowPlanXML>

Answer 1

您的表格被B分区。

除非另行指定，否则索引会继承此分区方案。例如

create unique index ix_ on T (A, B)  with (data_compression = page) on [primary]

（在这种情况下，它变得不对齐并阻止某些操作，例如仅切换元数据）

最低的＆＃34; A＆＃34;值可能在任何分区中。

这并未得到很好的优化。您可以保留对齐的索引并使用此重写based on the code here

SELECT TOP 20 A, B 
FROM sys.partitions AS P
  CROSS APPLY ( SELECT TOP 20 A, B 
                FROM dbo.T
                WHERE $PARTITION.YourPartitionFunction(T.B) = P.partition_number 
                ORDER BY A,B
                ) AS A
WHERE P.object_id = OBJECT_ID('dbo.T')
AND P.index_id = INDEXPROPERTY( OBJECT_ID('dbo.T'), 'ix_', 'IndexID' )
ORDER BY  A,B

它将获得41个分区中每个分区的前20行（没有排序），然后只排序由此产生的820行，以获得最后的前20名（而不是整个4200）。

为什么选择top ...按索引列的顺序仍然排序？

1 个答案: