说说sqlserver的执行计划

　　我们知道sql在底层的执行给我们上层人员开了一个窗口，那就是执行计划，有了执行计划之后，我们就清楚了那些烂sql是怎么执行的，这样

就可以方便的找到sql的缺陷和优化点。

一：执行计划生成过程

　　说到执行计划，首先要知道的是执行计划大概生成的过程，这样就可以做到就心中有数了，下面我画下简图：

1. 分析过程

　　这三个比较容易理解，首先我们要保证sql的语法不能错误，select和join的表是必须存在的，以及你是有执行这个sql的权限，对不对。。。

这样我们就走完了执行计划生命周期的第一个流程。

2. 编译过程

      保证了上面sql这三点的话，引擎就必须硬着头皮看你这么一大坨烂sql，该删的删，该改的改，该转换的转换，比如说你的“子查询”会转化为

“表连接”等等。。。其实也挺难为引擎的，举个例子吧。

<1>子查询生成的sql：

<2>join生成的sql：

从上面的两个结果中，你可以看到，大家都是玩join的，如果你仔细看的话，会发现一个是“哈希匹配”，一个是“嵌套循环”，为什么不一样，这

当然是引擎根据很多情况综合评选出来的，比如说：磁盘IO，逻辑读，资源占用，硬件环境等等。。。这也是所谓的“计划选优”操作。

3.执行过程

　　既然执行计划都选出来了，理所当然就要执行了，执行完后会把sql和执行计划放入缓存，这样下次有同样的sql过来的时候就可以直接从

Cache中提取了，不需要再次生成计划了，你也看到，生成执行计划还是比较消耗CPU时间的。

二：看看sql和执行的计划的缓存

　　刚才也说了，sql和plan都已经放入缓存了，那我的好奇心比较强，我就想看看sql和plan到底在哪，并且长的是个什么丑样子，刚好

sqlserver还是比较能够满足我们G点的。

1. 为了方便查看缓存，我需要先将所有的缓存清空，比如下面的语句。

DBCC freeproccache
SELECT c.* FROM dbo.Category AS c
JOIN dbo.Product AS p
ON c.CategoryId=p.CategoryId
WHERE c.CategoryId=23794

2. 通过sys.dm_exec_cached_plans拿到sql和plan的指针（plan_handle），如下图

SELECT * FROM sys.dm_exec_cached_plans

从图中你看到了两个adhoc（即时查询），分别是我在第一步执行的join查询和我在第二步执行的这个select。

3. 现在我们已经拿到了2个adhoc的plan_handle，然后通过dm_exec_sql_text查看他们的sql分别是怎样？

4. 看完text缓存，接下来我们继续看看sql的plan缓存在哪？可以通过dm_exec_query_plan来查看。

上面的query_plan字段就是所谓的执行计划，以xml的形式保存在字段中。。。所以说解析这个xml还是很费时间的。。。

  1 <?xml version="1.0"?>
  2 <ShowPlanXML xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan" Version="1.1" Build="10.0.1600.22">
  3     <BatchSequence>
  4         <Batch>
  5             <Statements>
  6                 <StmtSimple StatementText="SELECT c.* FROM dbo.Category AS c
  7  JOIN dbo.Product AS p
  8  ON c.CategoryId=p.CategoryId
  9  WHERE c.CategoryId=23794" StatementId="1" StatementCompId="1" StatementType="SELECT" StatementSubTreeCost="1.33278" 
  StatementEstRows="1.03803" StatementOptmLevel="FULL" QueryHash="0xB10B821B9B5E6396" QueryPlanHash="0x8C7B3B1660E28D16">
 10  <StatementSetOptions QUOTED_IDENTIFIER="true" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="true" ANSI_NULLS="true" 
 ANSI_PADDING="true" ANSI_WARNINGS="true" NUMERIC_ROUNDABORT="false" />
 11  <QueryPlan CachedPlanSize="16" CompileTime="2" CompileCPU="2" CompileMemory="168">
 12                         <MissingIndexes>
 13                             <MissingIndexGroup Impact="99.4633">
 14                                 <MissingIndex Database="[MYPETSHOP]" Schema="[dbo]" Table="[Product]">
 15                                     <ColumnGroup Usage="EQUALITY">
 16                                         <Column Name="[CategoryId]" ColumnId="2" />
 17                                     </ColumnGroup>
 18                                 </MissingIndex>
 19                             </MissingIndexGroup>
 20                             <MissingIndexGroup Impact="99.4636">
 21                                 <MissingIndex Database="[MYPETSHOP]" Schema="[dbo]" Table="[Product]">
 22                                     <ColumnGroup Usage="EQUALITY">
 23                                         <Column Name="[CategoryId]" ColumnId="2" />
 24                                     </ColumnGroup>
 25                                 </MissingIndex>
 26                             </MissingIndexGroup>
 27                         </MissingIndexes>
 28                         <RelOp NodeId="0" PhysicalOp="Nested Loops" LogicalOp="Inner Join" EstimateRows="1.03803" 
 EstimateIO="0" EstimateCPU="4.33898e-006" AvgRowSize="97" EstimatedTotalSubtreeCost="1.33278" Parallel="0" EstimateRebinds="0" 
 EstimateRewinds="0">
 29                             <OutputList>
 30                                 <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]" Table="[Category]" Alias="[c]" 
 Column="CategoryId" />
 31                                 <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]" Table="[Category]" Alias="[c]" 
 Column="Name" />
 32                                 <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]" Table="[Category]" Alias="[c]" 
 Column="Image" />
 33                             </OutputList>
 34                             <NestedLoops Optimized="0">
 35                                 <RelOp NodeId="1" PhysicalOp="Clustered Index Seek" LogicalOp="Clustered Index Seek" 
 EstimateRows="1" EstimateIO="0.003125" EstimateCPU="0.0001581" AvgRowSize="97" EstimatedTotalSubtreeCost="0.0032831" 
 TableCardinality="1.00001e+006" Parallel="0" EstimateRebinds="0" EstimateRewinds="0">
 36                                     <OutputList>
 37                                         <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]" Table="[Category]" 
 Alias="[c]" Column="CategoryId" />
 38                                         <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]" Table="[Category]" 
 Alias="[c]" Column="Name" />
 39                                         <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]" Table="[Category]"
  Alias="[c]" Column="Image" />
 40                                     </OutputList>
 41                                     <IndexScan Ordered="1" ScanDirection="FORWARD" ForcedIndex="0" ForceSeek="0"
  NoExpandHint="0">
 42                                         <DefinedValues>
 43                                             <DefinedValue>
 44                                                 <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]" 
 Table="[Category]" Alias="[c]" Column="CategoryId" />
 45                                             </DefinedValue>
 46                                             <DefinedValue>
 47                                                 <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]"
  Table="[Category]" Alias="[c]" Column="Name" />
 48                                             </DefinedValue>
 49                                             <DefinedValue>
 50                                                 <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]"
  Table="[Category]" Alias="[c]" Column="Image" />
 51                                             </DefinedValue>
 52                                         </DefinedValues>
 53                                         <Object Database="[MYPETSHOP]" Schema="[dbo]" Table="[Category]"
  Index="[PK_Category]" Alias="[c]" IndexKind="Clustered" />
 54                                         <SeekPredicates>
 55                                             <SeekPredicateNew>
 56                                                 <SeekKeys>
 57                                                     <Prefix ScanType="EQ">
 58                                                         <RangeColumns>
 59                                                             <ColumnReference Database="[MYPETSHOP]"
  Schema="[dbo]" Table="[Category]" Alias="[c]" Column="CategoryId" />
 60                                                         </RangeColumns>
 61                                                         <RangeExpressions>
 62                                                             <ScalarOperator ScalarString="(23794)">
 63                                                                 <Const ConstValue="(23794)" />
 64                                                             </ScalarOperator>
 65                                                         </RangeExpressions>
 66                                                     </Prefix>
 67                                                 </SeekKeys>
 68                                             </SeekPredicateNew>
 69                                         </SeekPredicates>
 70                                     </IndexScan>
 71                                 </RelOp>
 72                                 <RelOp NodeId="2" PhysicalOp="Clustered Index Scan" LogicalOp="Clustered Index Scan"
  EstimateRows="1.03803" EstimateIO="1.18831" EstimateCPU="0.0983419" AvgRowSize="11" EstimatedTotalSubtreeCost="1.28665" 
  TableCardinality="89259" Parallel="0" EstimateRebinds="0" EstimateRewinds="0">
 73                                     <OutputList />
 74                                     <IndexScan Ordered="0" ForcedIndex="0" NoExpandHint="0">
 75                                         <DefinedValues />
 76                                         <Object Database="[MYPETSHOP]" Schema="[dbo]" Table="[Product]" 
 Index="[PK_Product]" Alias="[p]" IndexKind="Clustered" />
 77                                         <Predicate>
 78  <ScalarOperator ScalarString="[MYPETSHOP].[dbo].[Product].[CategoryId] as [p].[CategoryId]=(23794)">
 79                                                 <Compare CompareOp="EQ">
 80                                                     <ScalarOperator>
 81                                                         <Identifier>
 82  <ColumnReference Database="[MYPETSHOP]" Schema="[dbo]" Table="[Product]" Alias="[p]" Column="CategoryId" />
 83                                                         </Identifier>
 84                                                     </ScalarOperator>
 85                                                     <ScalarOperator>
 86  <Const ConstValue="(23794)" />
 87                                                     </ScalarOperator>
 88                                                 </Compare>
 89                                             </ScalarOperator>
 90                                         </Predicate>
 91                                     </IndexScan>
 92                                 </RelOp>
 93                             </NestedLoops>
 94                         </RelOp>
 95                     </QueryPlan>
 96                 </StmtSimple>
 97             </Statements>
 98         </Batch>
 99     </BatchSequence>
100 </ShowPlanXML>

　　好了，到现在你应该认识到重新生成执行计划是不容易的。。。下一篇我们讨论讨论重用，重编译，重新生成等相关情况。