sql-server - INNER JOIN 和锁

Question

我有两个表TableA 和TableB，其中包含我想要同时检索和更新的信息。当我使用时

SELECT TOP 2 SomeFieldA
FROM TableA WITH (ROWLOCK , UPDLOCK , READPAST)

一切正常，进程 1 可以看到第 1 行和第 2 行，而进程 2 可以看到第 3 行和第 4 行。这是预期的行为。另外，当我执行 EXEC sp_lock 时，我只看到两个 KEY 条目。但是，当我将语句更改为

SELECT TOP 2 SomeFieldA
FROM TableA WITH (ROWLOCK , UPDLOCK , READPAST)
INNER JOIN
Table B WITH ( ROWLOCK , UPDLOCK , READPAST )
ON TableA.ID = TableB.IDRef`

第一个进程看到第 1 行和第 2 行，但进程 2 什么也看不到。执行 sp_lock 显示现在所有行都已被阻止。为什么会发生这种情况？

编辑: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="1.1" Build="10.50.1617.0" xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan">
  <BatchSequence>
    <Batch>
      <Statements>
        <StmtSimple StatementCompId="2" StatementEstRows="2" StatementId="1" StatementOptmLevel="FULL" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" StatementSubTreeCost="0.00670141" StatementText="SELECT TOP 2 * FROM Request R WITH ( ROWLOCK , UPDLOCK , READPAST) INNER JOIN Options O WITH ( ROWLOCK , UPDLOCK , READPAST ) ON (R.RequestID = O.RequestID)&#xD;&#xA;&#xD;" StatementType="SELECT" QueryHash="0xA35BE09F9DD52334" QueryPlanHash="0x95BEDE8C14AB4C68">
          <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" CachedPlanSize="16" CompileTime="3" CompileCPU="3" CompileMemory="160">
            <RelOp AvgRowSize="58" EstimateCPU="2E-07" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="2" LogicalOp="Top" NodeId="0" Parallel="false" PhysicalOp="Top" EstimatedTotalSubtreeCost="0.00670141">
              <OutputList>
                <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="RequestID" />
                <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="DateEntered" />
                <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="Priority" />
                <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Alias="[O]" Column="RequestID" />
                <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Alias="[O]" Column="SomeOptions" />
              </OutputList>
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRows="2" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <Top RowCount="false" IsPercent="false" WithTies="false">
                <TopExpression>
                  <ScalarOperator ScalarString="(2)">
                    <Const ConstValue="(2)" />
                  </ScalarOperator>
                </TopExpression>
                <RelOp AvgRowSize="58" EstimateCPU="7.524E-05" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="2" LogicalOp="Inner Join" NodeId="1" Parallel="false" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="0.00670121">
                  <OutputList>
                    <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="RequestID" />
                    <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="DateEntered" />
                    <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="Priority" />
                    <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Alias="[O]" Column="RequestID" />
                    <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Alias="[O]" Column="SomeOptions" />
                  </OutputList>
                  <RunTimeInformation>
                    <RunTimeCountersPerThread Thread="0" ActualRows="2" ActualEndOfScans="0" ActualExecutions="1" />
                  </RunTimeInformation>
                  <NestedLoops Optimized="false">
                    <Predicate>
                      <ScalarOperator ScalarString="[TestDatabase].[dbo].[Options].[RequestID] as [O].[RequestID]=[TestDatabase].[dbo].[Request].[RequestID] as [R].[RequestID]">
                        <Compare CompareOp="EQ">
                          <ScalarOperator>
                            <Identifier>
                              <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Alias="[O]" Column="RequestID" />
                            </Identifier>
                          </ScalarOperator>
                          <ScalarOperator>
                            <Identifier>
                              <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="RequestID" />
                            </Identifier>
                          </ScalarOperator>
                        </Compare>
                      </ScalarOperator>
                    </Predicate>
                    <RelOp AvgRowSize="45" EstimateCPU="0.0001603" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="2" LogicalOp="Clustered Index Scan" NodeId="2" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032842" TableCardinality="3">
                      <OutputList>
                        <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Alias="[O]" Column="RequestID" />
                        <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Alias="[O]" Column="SomeOptions" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="0" ActualRows="2" ActualEndOfScans="0" ActualExecutions="1" />
                      </RunTimeInformation>
                      <IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                        <DefinedValues>
                          <DefinedValue>
                            <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Alias="[O]" Column="RequestID" />
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Alias="[O]" Column="SomeOptions" />
                          </DefinedValue>
                        </DefinedValues>
                        <Object Database="[TestDatabase]" Schema="[dbo]" Table="[Options]" Index="[PK__Options__33A8519A1DE57479]" Alias="[O]" IndexKind="Clustered" />
                      </IndexScan>
                    </RelOp>
                    <RelOp AvgRowSize="20" EstimateCPU="8.51E-05" EstimateIO="0.0032035" EstimateRebinds="0" EstimateRewinds="1.33333" EstimateRows="6" LogicalOp="Table Scan" NodeId="3" Parallel="false" PhysicalOp="Table Scan" EstimatedTotalSubtreeCost="0.00340207" TableCardinality="6">
                      <OutputList>
                        <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="RequestID" />
                        <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="DateEntered" />
                        <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="Priority" />
                      </OutputList>
                      <RunTimeInformation>
                        <RunTimeCountersPerThread Thread="0" ActualRows="8" ActualEndOfScans="1" ActualExecutions="2" />
                      </RunTimeInformation>
                      <TableScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
                        <DefinedValues>
                          <DefinedValue>
                            <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="RequestID" />
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="DateEntered" />
                          </DefinedValue>
                          <DefinedValue>
                            <ColumnReference Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" Column="Priority" />
                          </DefinedValue>
                        </DefinedValues>
                        <Object Database="[TestDatabase]" Schema="[dbo]" Table="[Request]" Alias="[R]" IndexKind="Heap" />
                      </TableScan>
                    </RelOp>
                  </NestedLoops>
                </RelOp>
              </Top>
            </RelOp>
          </QueryPlan>
        </StmtSimple>
      </Statements>
    </Batch>
  </BatchSequence>
</ShowPlanXML>

SQL:

CREATE TABLE Request
(
    RequestID   INT PRIMARY KEY,
    Priority    INT,
    DateEntered DATETIME
)

CREATE TABLE Options
(
    RequestIDRef    INT PRIMARY KEY,
    SomeOptions     NVARCHAR(MAX)
)
ALTER TABLE Options ADD

    CONSTRAINT FK_REQUESTIDREF FOREIGN KEY ( RequestIDRef ) REFERENCES [Request] ( RequestID )

GO

INSERT INTO Request VALUES ( 1, 2, GETDATE() )
INSERT INTO Request VALUES ( 2, 1, GETDATE() )
INSERT INTO Request VALUES ( 3, 3, GETDATE() )
INSERT INTO Request VALUES ( 4, 2, GETDATE() )

INSERT INTO Options VALUES ( 1, 'a' )
INSERT INTO Options VALUES ( 2, 'b' )
INSERT INTO Options VALUES ( 3, 'c' )
INSERT INTO Options VALUES ( 4, 'd' )

CREATE NONCLUSTERED INDEX IX_REQUESTIDREF ON [Options] ( RequestIDRef )
CREATE NONCLUSTERED INDEX IX_PRIORITY_DATEENTERED ON [Request] ( Priority , DateEntered ) INCLUDE ( RequestID )

现在，

BEGIN TRANSACTION

SELECT TOP 2 * FROM [Request] WITH ( ROWLOCK , UPDLOCK , READPAST ) INNER JOIN [Options] WITH ( ROWLOCK , UPDLOCK , READPAST ) ON ( Request.RequestID = Options.RequestIDRef ) ORDER BY Priority, DateEntered

WAITFOR DELAY '00:00:02.5'

COMMIT TRANSACTION

按预期，在 Query1 上返回 2 和 1，但在 Query2 上它什么也不返回。但是，如果我删除 INNER JOIN 和第二个表，它就会起作用并在 Query1 中返回 (2,1)，在 Query2 中返回 (3,4)。

Answer 1

逻辑上，A 和 B 的笛卡尔积仅限于交集或匹配行。

为了找到这些匹配的行，需要查看 TableA.ID 和 TableB.IDRef。如果没有索引，则至少需要对其中一个表进行表扫描。因此所有行都被锁定以进行扫描

所以TableA.ID和TableB.IDRef都需要有索引。我怀疑 TableA.ID 已经具有 PK，但 TableB.IDRef 没有。

顺便说一句，TOP 稍后应用

这与我的答案类似，其中有 TOP，没有 ORDER BY，也没有索引: ORDER BY and WITH(ROWLOCK, UPDLOCK, READPAST) 。如果您将 ORDER BY 添加到第一个查询，那么第二个进程很可能不会看到任何内容。

编辑:对于您的更新，SELECT * 将使索引使用无效并导致扫描:索引与 SELECT * 没有多大用处，因为它们没有覆盖