https://stackoverflow.com/questions/17580830
italiano
english
français
española
中国
日本の
العربية
Deutsch
한국어
Português
RussianPer Mikael's comment. The first query is faster but has a higher cost because it uses parallelism.
SQL Server Query Plans, Elapsed Time is Lower, but Subtree Cost is Higher
-
02-06-2022 - |
Вопрос
Why is the elapsed execution time of first of the two queries below lower, while the estimated subtree cost is higher?
Estimated subtree cost is only a guideline for gauging query plan performance in conjunction with other query performance indicators, like response time, but I'm surprised by the existance of an inverse relationship between cost and time when comparing two similar queries side by side.
I've created an example database to illustrate this problem and included the resulting XML query plans for both queries. I realize the existence of the StateID as a foreign key in both the County and Town tables is bad form, but it was the easiest way for me to illustrate this phenomenon.
I'm not just asking "which is more important, subtree cost or response time." I want to understand how an inverse relationship can exist between query plan subtree cost and response time for two similar queries, so that I can make more informed decisions going forward.
Please note that the two queries below produce the exact same result set.
Thanks!
CREATE TABLE dbo.County (StateID INT, CountyID INT)
CREATE TABLE dbo.Town (StateID INT, CountyID INT, TownID INT)
DECLARE @State TABLE (StateID INT, PRIMARY KEY CLUSTERED (StateID))
DECLARE @County TABLE (CountyID INT, PRIMARY KEY CLUSTERED (CountyID))
INSERT INTO @State
VALUES (27)
INSERT INTO @County
SELECT DISTINCT
c.CountyID
FROM dbo.County c
JOIN @State s ON c.StateID = s.StateID
-- low time, high cost query
SELECT DISTINCT
t.CountyID,
t.TownID
FROM dbo.Town t
JOIN @State s
ON t.StateID = s.StateID
-- low cost, high time query
SELECT DISTINCT
t.CountyID,
t.TownID
FROM dbo.Town t
JOIN @State s
ON t.StateID = s.StateID
JOIN @County c
ON t.CountyID = c.CountyID
The following query plan is for the query that had an estimated subtree cost of 6.06 and an elapsed time of 1114 milliseconds.
<StmtSimple StatementCompId="6" StatementEstRows="273495" StatementId="3" StatementOptmLevel="FULL" StatementSubTreeCost="6.06304" StatementText="SELECT DISTINCT
	t.CountyID,
	t.TownID
FROM dbo.Town t
JOIN @State s
	ON t.StateID = s.StateID

-- low cost, high time query
" StatementType="SELECT" QueryHash="0x9347C19165C31DBF" QueryPlanHash="0x98EE57C66D8B347A">
<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="2" MemoryGrant="28144" CachedPlanSize="32" CompileTime="8" CompileCPU="8" CompileMemory="312">
<RelOp AvgRowSize="15" EstimateCPU="0.388488" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Gather Streams" NodeId="0" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="6.06304">
<OutputList>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="13027" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<Parallelism>
<RelOp AvgRowSize="15" EstimateCPU="3.79725" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Aggregate" NodeId="1" Parallel="true" PhysicalOp="Hash Match" EstimatedTotalSubtreeCost="5.67455">
<OutputList>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OutputList>
<MemoryFractions Input="0.998723" Output="1" />
<RunTimeInformation>
<RunTimeCountersPerThread Thread="2" ActualRows="6515" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="1" ActualRows="6512" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
</RunTimeInformation>
<Hash>
<DefinedTowns />
<HashKeysBuild>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</HashKeysBuild>
<BuildResidual>
<ScalarOperator ScalarString="[Locality].[dbo].[Town].[CountyID] as [t].[CountyID] = [Locality].[dbo].[Town].[CountyID] as [t].[CountyID] AND [Locality].[dbo].[Town].[TownID] as [t].[TownID] = [Locality].[dbo].[Town].[TownID] as [t].[TownID]">
<Logical Operation="AND">
<ScalarOperator>
<Compare CompareOp="IS">
<ScalarOperator>
<Identifier>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Identifier>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
</Identifier>
</ScalarOperator>
</Compare>
</ScalarOperator>
<ScalarOperator>
<Compare CompareOp="IS">
<ScalarOperator>
<Identifier>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Identifier>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</Identifier>
</ScalarOperator>
</Compare>
</ScalarOperator>
</Logical>
</ScalarOperator>
</BuildResidual>
<RelOp AvgRowSize="15" EstimateCPU="0.494228" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Repartition Streams" NodeId="2" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="1.8773">
<OutputList>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="2" ActualRows="1017328" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="1" ActualRows="1093191" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
</RunTimeInformation>
<Parallelism PartitioningType="Hash">
<PartitionColumns>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</PartitionColumns>
<RelOp AvgRowSize="15" EstimateCPU="0.571604" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Inner Join" NodeId="3" Parallel="true" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="1.38307">
<OutputList>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="2" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="1" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
</RunTimeInformation>
<NestedLoops Optimized="true">
<OuterReferences>
<ColumnReference Table="@State" Alias="[s]" Column="StateID" />
</OuterReferences>
<RelOp AvgRowSize="11" EstimateCPU="0.0285019" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Distribute Streams" NodeId="5" Parallel="true" PhysicalOp="Parallelism" EstimatedTotalSubtreeCost="0.031785">
<OutputList>
<ColumnReference Table="@State" Alias="[s]" Column="StateID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="2" ActualRows="0" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="1" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
</RunTimeInformation>
<Parallelism PartitioningType="RoundRobin">
<RelOp AvgRowSize="11" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Clustered Index Scan" NodeId="6" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="0">
<OutputList>
<ColumnReference Table="@State" Alias="[s]" Column="StateID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="1" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
</RunTimeInformation>
<IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
<DefinedTowns>
<DefinedTown>
<ColumnReference Table="@State" Alias="[s]" Column="StateID" />
</DefinedTown>
</DefinedTowns>
<Object Table="[@State]" Index="[PK__#3AB788A__C3BA3B5A3C9FD11A]" Alias="[s]" />
</IndexScan>
</RelOp>
</Parallelism>
</RelOp>
<RelOp AvgRowSize="15" EstimateCPU="0.301001" EstimateIO="0.478681" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="273495" LogicalOp="Index Seek" NodeId="7" Parallel="true" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.779682" TableCardinality="6291320">
<OutputList>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="2" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
<RunTimeCountersPerThread Thread="1" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
<RunTimeCountersPerThread Thread="0" ActualRows="0" ActualEndOfScans="0" ActualExecutions="0" />
</RunTimeInformation>
<IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" ForceSeek="false" NoExpandHint="false">
<DefinedTowns>
<DefinedTown>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
</DefinedTown>
<DefinedTown>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</DefinedTown>
</DefinedTowns>
<Object Database="[Locality]" Schema="[dbo]" Table="[Town]" Index="[NCX_Town_StateID_CountyID_TownID]" Alias="[t]" IndexKind="NonClustered" />
<SeekPredicates>
<SeekPredicateNew>
<SeekKeys>
<Prefix ScanType="EQ">
<RangeColumns>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
</RangeColumns>
<RangeExpressions>
<ScalarOperator ScalarString="@State.[StateID] as [s].[StateID]">
<Identifier>
<ColumnReference Table="@State" Alias="[s]" Column="StateID" />
</Identifier>
</ScalarOperator>
</RangeExpressions>
</Prefix>
</SeekKeys>
</SeekPredicateNew>
</SeekPredicates>
</IndexScan>
</RelOp>
</NestedLoops>
</RelOp>
</Parallelism>
</RelOp>
</Hash>
</RelOp>
</Parallelism>
</RelOp>
</QueryPlan>
</StmtSimple>
The following query plan is for the query that had an estimated subtree cost of 0.14 and an elapsed time of 4614 milliseconds.
<StmtSimple StatementCompId="7" StatementEstRows="339.324" StatementId="4" StatementOptmLevel="FULL" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" StatementSubTreeCost="0.142839" StatementText="SELECT DISTINCT
	t.CountyID,
	t.TownID
FROM dbo.Town t
JOIN @State s
	ON t.StateID = s.StateID
JOIN @County c
	ON t.CountyID = c.CountyID

" StatementType="SELECT" QueryHash="0xF7B13FA8059B141C" QueryPlanHash="0x31CEE9DAF12E77A5">
<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" MemoryGrant="1584" CachedPlanSize="40" CompileTime="5" CompileCPU="5" CompileMemory="376">
<RelOp AvgRowSize="15" EstimateCPU="0.00454996" EstimateIO="0.0112613" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="339.324" LogicalOp="Distinct Sort" NodeId="0" Parallel="false" PhysicalOp="Sort" EstimatedTotalSubtreeCost="0.142839">
<OutputList>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OutputList>
<MemoryFractions Input="1" Output="1" />
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRebinds="1" ActualRewinds="0" ActualRows="13027" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<Sort Distinct="true">
<OrderBy>
<OrderByColumn Ascending="true">
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
</OrderByColumn>
<OrderByColumn Ascending="true">
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OrderByColumn>
</OrderBy>
<RelOp AvgRowSize="15" EstimateCPU="0.0571741" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="339.324" LogicalOp="Inner Join" NodeId="1" Parallel="false" PhysicalOp="Hash Match" EstimatedTotalSubtreeCost="0.127028">
<OutputList>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OutputList>
<MemoryFractions Input="0" Output="0" />
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<Hash>
<DefinedTowns />
<HashKeysBuild>
<ColumnReference Table="@State" Alias="[s]" Column="StateID" />
</HashKeysBuild>
<HashKeysProbe>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
</HashKeysProbe>
<RelOp AvgRowSize="11" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Clustered Index Scan" NodeId="2" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="0">
<OutputList>
<ColumnReference Table="@State" Alias="[s]" Column="StateID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
<DefinedTowns>
<DefinedTown>
<ColumnReference Table="@State" Alias="[s]" Column="StateID" />
</DefinedTown>
</DefinedTowns>
<Object Table="[@State]" Index="[PK__#3AB788A__C3BA3B5A3C9FD11A]" Alias="[s]" />
</IndexScan>
</RelOp>
<RelOp AvgRowSize="19" EstimateCPU="0.0357739" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="8558.34" LogicalOp="Inner Join" NodeId="3" Parallel="false" PhysicalOp="Nested Loops" EstimatedTotalSubtreeCost="0.066568">
<OutputList>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="2110519" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<NestedLoops Optimized="false">
<OuterReferences>
<ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
</OuterReferences>
<RelOp AvgRowSize="11" EstimateCPU="0.0001581" EstimateIO="0.003125" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Clustered Index Scan" NodeId="4" Parallel="false" PhysicalOp="Clustered Index Scan" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="0">
<OutputList>
<ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="137" ActualEndOfScans="1" ActualExecutions="1" />
</RunTimeInformation>
<IndexScan Ordered="false" ForcedIndex="false" NoExpandHint="false">
<DefinedTowns>
<DefinedTown>
<ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
</DefinedTown>
</DefinedTowns>
<Object Table="[@County]" Index="[PK__#3E88198__B68F9DF7407061FE]" Alias="[c]" />
</IndexScan>
</RelOp>
<RelOp AvgRowSize="19" EstimateCPU="0.00957118" EstimateIO="0.0179398" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="8558.34" LogicalOp="Index Seek" NodeId="5" Parallel="false" PhysicalOp="Index Seek" EstimatedTotalSubtreeCost="0.027511" TableCardinality="6291320">
<OutputList>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</OutputList>
<RunTimeInformation>
<RunTimeCountersPerThread Thread="0" ActualRows="2110519" ActualEndOfScans="137" ActualExecutions="137" />
</RunTimeInformation>
<IndexScan Ordered="true" ScanDirection="FORWARD" ForcedIndex="false" ForceSeek="false" NoExpandHint="false">
<DefinedTowns>
<DefinedTown>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="StateID" />
</DefinedTown>
<DefinedTown>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
</DefinedTown>
<DefinedTown>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="TownID" />
</DefinedTown>
</DefinedTowns>
<Object Database="[Locality]" Schema="[dbo]" Table="[Town]" Index="[NCX_Town_CountyID_inc_StateID_TownID]" Alias="[t]" IndexKind="NonClustered" />
<SeekPredicates>
<SeekPredicateNew>
<SeekKeys>
<Prefix ScanType="EQ">
<RangeColumns>
<ColumnReference Database="[Locality]" Schema="[dbo]" Table="[Town]" Alias="[t]" Column="CountyID" />
</RangeColumns>
<RangeExpressions>
<ScalarOperator ScalarString="@County.[CountyID] as [c].[CountyID]">
<Identifier>
<ColumnReference Table="@County" Alias="[c]" Column="CountyID" />
</Identifier>
</ScalarOperator>
</RangeExpressions>
</Prefix>
</SeekKeys>
</SeekPredicateNew>
</SeekPredicates>
</IndexScan>
</RelOp>
</NestedLoops>
</RelOp>
</Hash>
</RelOp>
</Sort>
</RelOp>
</QueryPlan>
</StmtSimple>
Решение 2
Другие советы
While I haven't looked too deeply into your query plans, I have advice and a suggested explanation. The advice is simple: do not use table variables for tables with more than two orders of magnitude of rows (100 rows). The possible explanation is that their performance can be very poor above that and the query plans generated can be inaccurate or inefficient, per Microsoft's article on table data types:
Best practices
Do not use table variables to store large amounts of data (more than 100 rows). Plan choices may not be optimal or stable when a table variable contains a large amount of data. Consider rewriting such queries to use temporary tables or use the USE PLAN query hint to ensure the optimizer uses an existing query plan that works well for your scenario.
And:
Limitations and Restrictions
table variables are not supported in the SQL Server optimizer's cost-based reasoning model. Therefore, they should not be used when cost-based choices are required to achieve an efficient query plan. Temporary tables are preferred when cost-based choices are required. This typically includes queries with joins, parallelism decisions, and index selection choices.
Queries that modify table variables do not generate parallel query execution plans. Performance can be affected when very large table variables, or table variables in complex queries, are modified. In these situations, consider using temporary tables instead. For more information, see CREATE TABLE (Transact-SQL). Queries that read table variables without modifying them can still be parallelized.
...
In practice, I've found that using table variables precludes generation of a parallel query plan, and hurts performance for all but the simplest, least complex table variables.
Instead, I find temporary tables to be more performant in almost every case, and as long as you are careful to clean up after yourself in a timely manner (or use short-lived sessions to allow the server to clear the temporary tables) then you get vastly better performance.
As to why the second query plan has a lower cost but higher execution time? It's likely that there are many, many counties (1000s? 10000s?) and the query plan being generated is assuming there are a small number (100s). The query optimizer thinks @County is likely to have less than 100 rows, and so it likely thinks that the result set will be very specific, because a list of towns joined with one list of <100 entries and then joined with a second list of <100 entries is, well, likely to have a lot fewer than 100 rows.
In reality, every town has a county, so the join isn't very selective and poor choices for types of joins are used. In each case, a nested loop was used for the last join, but a nested loop join is extremely inefficient for large, sorted data. And this is likely the crux of the issue.
Rerun your queries using temporary tables instead of table variables and I am guessing you will get very different query plans with very different performance characteristics.
Не связан с StackOverflow
