I have found interesting "issue" when using ADO driver with sql server 2008 (or rather when using specific sql cursors that are created by the driver). We want to execute following query:
SELECT ID, TelEveID, TelDayID, TelMobID, QualityFlag, WeekBegin FROM CEF
WHERE (TelEveID = '+44xxxx') OR (TelDayID = '+44xxxxx') OR (TelMobID = '+44xxxx')
ORDER BY ID DESC
where there are (for the sake of example) three indexed on CEF table for tuples [ID; TelEveID], [ID; TelDayID], and [ID; TelMobID]
the query above is "translated" by the driver in following way:
declare @p1 int
declare @p2 int
declare @p5 int
set @p5=4
declare @p6 int
set @p6=4
declare @p7 int
set @p7=-1
exec sp_cursorprepexec @p1 output,@p2 output,NULL,N'SELECT ID, TelEveID, TelDayID, TelMobID, QualityFlag, WeekBegin FROM CEF WHERE (TelEveID = ''+44xxxx'') OR (TelDayID = ''+44xxxx'') OR (TelMobID = ''+44xxxx'') ORDER BY ID DESC',@p5 output,@p6 output,@p7 output
exec sp_cursorfetch @p2,2,1,10
The problem is that if you execute the queries above first one is approx. 10 times faster than the second one. AND if we have a look at the actual execution plan we can see the reason for the difference
Cursor completely omits existing indexes and it uses Index Scan instead whereas "direct query" merges three "separate" resultsets into the final one with appropriate index usage.
If I change "WHERE" part within the cursor definition so it looks into one field only, it will use the appropriate index (it is still slower due to the cursor's running costs).
Is there any way how to avoid this behavior without removing the cursors completely (as this is not possible in the legacy application that performs this action)?
Thanks
Martin F.
this is (little bit shortened - i have removed information about all the columns in CEF table) (xml) execution plan for the cursor base query:
<?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="6" StatementEstRows="3.3163" StatementId="1" StatementOptmLevel="FULL" StatementSubTreeCost="134.243" StatementText="SELECT ID, TelEveID, TelDayID, TelMobID, QualityFlag, WeekBegin FROM CEF WHERE (TelEveID = '+44xxxx') OR (TelDayID = '+44xxxx') OR (TelMobID = '+44xxxx') ORDER BY ID DESC" StatementType="DECLARE CURSOR" QueryHash="0x4DC45EB62BCCAE06" QueryPlanHash="0x72ACC7B98E06E06E">
<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="0" CachedPlanSize="152" CompileTime="7" CompileCPU="3" CompileMemory="992">
<RelOp AvgRowSize="913" EstimateCPU="3.3163E-06" EstimateIO="0.01" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="3.3163" LogicalOp="Insert" NodeId="0" Parallel="false" PhysicalOp="Clustered Index Insert" EstimatedTotalSubtreeCost="134.243">
<OutputList>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
<ColumnReference Column="Expr1005" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="1" />
</RunTimeInformation>
<Update DMLRequestSort="false">
<Object Database="[tempdb]" Index="[CWT_PrimaryKey]" />
<SetPredicate>
<ScalarOperator ScalarString="[CWT].[COLUMN0] = [LMS].[dbo].[CEF].[ID],[CWT].[CHECKSUM1] = [Chk1002],[CWT].[ROWID] = [Expr1005]">
<ScalarExpressionList>
<ScalarOperator>
<MultipleAssign>
<Assign>
<ColumnReference Table="[CWT]" Column="COLUMN0" />
<ScalarOperator>
<Identifier>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
</Identifier>
</ScalarOperator>
</Assign>
<Assign>
<ColumnReference Table="[CWT]" Column="CHECKSUM1" />
<ScalarOperator>
<Identifier>
<ColumnReference Column="Chk1002" />
</Identifier>
</ScalarOperator>
</Assign>
<Assign>
<ColumnReference Table="[CWT]" Column="ROWID" />
<ScalarOperator>
<Identifier>
<ColumnReference Column="Expr1005" />
</Identifier>
</ScalarOperator>
</Assign>
</MultipleAssign>
</ScalarOperator>
</ScalarExpressionList>
</ScalarOperator>
</SetPredicate>
<RelOp AvgRowSize="917" EstimateCPU="3.3163E-07" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="3.3163" LogicalOp="Compute Scalar" NodeId="1" Parallel="false" PhysicalOp="Compute Scalar" EstimatedTotalSubtreeCost="134.233">
<OutputList>
<ColumnReference Column="Chk1002" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
<ColumnReference Column="Expr1005" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="1" />
</RunTimeInformation>
<ComputeScalar>
<DefinedValues>
<DefinedValue>
<ColumnReference Column="Expr1005" />
<ScalarOperator ScalarString="CWT_ROWID()">
<Intrinsic FunctionName="CWT_ROWID" />
</ScalarOperator>
</DefinedValue>
</DefinedValues>
<RelOp AvgRowSize="913" EstimateCPU="1.68105" EstimateIO="130.748" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="3.3163" LogicalOp="Clustered Index Scan" NodeId="2" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="132.429" TableCardinality="1528090">
<OutputList>
<ColumnReference Column="Chk1002" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="1" />
</RunTimeInformation>
<IndexScan Ordered="true" ScanDirection="BACKWARD" ForcedIndex="false" ForceSeek="false" NoExpandHint="false">
<DefinedValues>
<DefinedValue>
<ColumnReference Column="Chk1002" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
</DefinedValue>
</DefinedValues>
<Object Database="[LMS]" Schema="[dbo]" Table="[CEF]" Index="[PK_CEF]" IndexKind="Clustered" />
<Predicate>
<ScalarOperator ScalarString="[LMS].[dbo].[CEF].[TelEveID]='+44xxxx' OR [LMS].[dbo].[CEF].[TelDayID]='+44xxxx' OR [LMS].[dbo].[CEF].[TelMobID]='+44xxxx'">
<Logical Operation="OR">
<ScalarOperator>
<Compare CompareOp="EQ">
<ScalarOperator>
<Identifier>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Const ConstValue="'+44xxxx'" />
</ScalarOperator>
</Compare>
</ScalarOperator>
<ScalarOperator>
<Compare CompareOp="EQ">
<ScalarOperator>
<Identifier>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Const ConstValue="'+44xxxx'" />
</ScalarOperator>
</Compare>
</ScalarOperator>
<ScalarOperator>
<Compare CompareOp="EQ">
<ScalarOperator>
<Identifier>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Const ConstValue="'+44xxxx'" />
</ScalarOperator>
</Compare>
</ScalarOperator>
</Logical>
</ScalarOperator>
</Predicate>
</IndexScan>
</RelOp>
</ComputeScalar>
</RelOp>
</Update>
</RelOp>
</QueryPlan>
</StmtSimple>
</Statements>
</Batch>
<Batch>
<Statements>
<StmtSimple StatementCompId="7" StatementEstRows="3.3163" StatementId="2" StatementOptmLevel="FULL" StatementSubTreeCost="134.243" StatementText="FETCH API_CURSOR000000000007AA17" StatementType="FETCH CURSOR" QueryHash="0x4DC45EB62BCCAE06" QueryPlanHash="0x72ACC7B98E06E06E">
<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="0" CachedPlanSize="152" CompileTime="7" CompileCPU="3" CompileMemory="992">
<RelOp AvgRowSize="913" EstimateCPU="3.3163E-06" EstimateIO="0.01" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="3.3163" LogicalOp="Insert" NodeId="0" Parallel="false" PhysicalOp="Clustered Index Insert" EstimatedTotalSubtreeCost="134.243">
<OutputList>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
<ColumnReference Column="Expr1005" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<Update DMLRequestSort="false">
<Object Database="[tempdb]" Index="[CWT_PrimaryKey]" />
<SetPredicate>
<ScalarOperator ScalarString="[CWT].[COLUMN0] = [LMS].[dbo].[CEF].[ID],[CWT].[CHECKSUM1] = [Chk1002],[CWT].[ROWID] = [Expr1005]">
<ScalarExpressionList>
<ScalarOperator>
<MultipleAssign>
<Assign>
<ColumnReference Table="[CWT]" Column="COLUMN0" />
<ScalarOperator>
<Identifier>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
</Identifier>
</ScalarOperator>
</Assign>
<Assign>
<ColumnReference Table="[CWT]" Column="CHECKSUM1" />
<ScalarOperator>
<Identifier>
<ColumnReference Column="Chk1002" />
</Identifier>
</ScalarOperator>
</Assign>
<Assign>
<ColumnReference Table="[CWT]" Column="ROWID" />
<ScalarOperator>
<Identifier>
<ColumnReference Column="Expr1005" />
</Identifier>
</ScalarOperator>
</Assign>
</MultipleAssign>
</ScalarOperator>
</ScalarExpressionList>
</ScalarOperator>
</SetPredicate>
<RelOp AvgRowSize="917" EstimateCPU="3.3163E-07" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="3.3163" LogicalOp="Compute Scalar" NodeId="1" Parallel="false" PhysicalOp="Compute Scalar" EstimatedTotalSubtreeCost="134.233">
<OutputList>
<ColumnReference Column="Chk1002" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
<ColumnReference Column="Expr1005" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<ComputeScalar>
<DefinedValues>
<DefinedValue>
<ColumnReference Column="Expr1005" />
<ScalarOperator ScalarString="CWT_ROWID()">
<Intrinsic FunctionName="CWT_ROWID" />
</ScalarOperator>
</DefinedValue>
</DefinedValues>
<RelOp AvgRowSize="913" EstimateCPU="1.68105" EstimateIO="130.748" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="3.3163" LogicalOp="Clustered Index Scan" NodeId="2" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="132.429" TableCardinality="1528090">
<OutputList>
<ColumnReference Column="Chk1002" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<IndexScan Ordered="true" ScanDirection="BACKWARD" ForcedIndex="false" ForceSeek="false" NoExpandHint="false">
<DefinedValues>
<DefinedValue>
<ColumnReference Column="Chk1002" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="ID" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
</DefinedValue>
<DefinedValue>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
</DefinedValue>
</DefinedValues>
<Object Database="[LMS]" Schema="[dbo]" Table="[CEF]" Index="[PK_CEF]" IndexKind="Clustered" />
<Predicate>
<ScalarOperator ScalarString="[LMS].[dbo].[CEF].[TelEveID]='+44xxxx' OR [LMS].[dbo].[CEF].[TelDayID]='+44xxxx' OR [LMS].[dbo].[CEF].[TelMobID]='+44xxxx'">
<Logical Operation="OR">
<ScalarOperator>
<Compare CompareOp="EQ">
<ScalarOperator>
<Identifier>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelEveID" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Const ConstValue="'+44xxxx'" />
</ScalarOperator>
</Compare>
</ScalarOperator>
<ScalarOperator>
<Compare CompareOp="EQ">
<ScalarOperator>
<Identifier>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelDayID" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Const ConstValue="'+44xxxx'" />
</ScalarOperator>
</Compare>
</ScalarOperator>
<ScalarOperator>
<Compare CompareOp="EQ">
<ScalarOperator>
<Identifier>
<ColumnReference Database="[LMS]" Schema="[dbo]" Table="[CEF]" Column="TelMobID" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Const ConstValue="'+44xxxx'" />
</ScalarOperator>
</Compare>
</ScalarOperator>
</Logical>
</ScalarOperator>
</Predicate>
</IndexScan>
</RelOp>
</ComputeScalar>
</RelOp>
</Update>
</RelOp>
</QueryPlan>
</StmtSimple>
</Statements>
</Batch>
</BatchSequence>
</ShowPlanXML>