SQL Server 2022

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CREATE TABLE TableA (
  "ColumnA" VARCHAR(1),
  "UserId" INTEGER PRIMARY KEY
);

INSERT INTO TableA
  ("ColumnA", "UserId")
VALUES
  ('x', '1'),
  ('y', '2'),
  ('z', '3'),
  ('w', '4');

CREATE TABLE TableB (
  "ColumnB" VARCHAR(1),
  "UserId" INTEGER PRIMARY KEY
);

INSERT INTO TableB
  ("ColumnB", "UserId")
VALUES
  ('a', '1'),
  ('b', '3'),
  ('c', '5'),
  ('d', '6');
8 rows affected

set statistics xml on
SELECT TOP (3)
  t.UserId
FROM (
    SELECT a.UserId
    FROM TableA a
    UNION ALL
    SELECT b.UserId
    FROM TableB b
) t
GROUP BY t.UserId
ORDER BY UserId;


UserId


1

2

3




Microsoft SQL Server 2005 XML Showplan


<ShowPlanXML xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan" Version="1.564" Build="16.0.1000.6"><BatchSequence><Batch><Statements><StmtSimple StatementText="SELECT TOP (3)&#xa;  t.UserId&#xa;FROM (&#xa;    SELECT a.UserId&#xa;    FROM TableA a&#xa;    UNION ALL&#xa;    SELECT b.UserId&#xa;    FROM TableB b&#xa;) t&#xa;GROUP BY t.UserId&#xa;ORDER BY UserId" StatementId="1" StatementCompId="1" StatementType="SELECT" StatementSqlHandle="0x0900B4484F23BC372CB2DEAFF4EEF22A004F0000000000000000000000000000000000000000000000000000" DatabaseContextSettingsId="1" ParentObjectId="0" StatementParameterizationType="0" RetrievedFromCache="true" StatementSubTreeCost="0.0121842" StatementEstRows="3" SecurityPolicyApplied="false" StatementOptmLevel="FULL" QueryHash="0x5083708FF924F1AA" QueryPlanHash="0x1B2CC007C3491057" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" CardinalityEstimationModelVersion="160"><StatementSetOptions QUOTED_IDENTIFIER="true" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="true" ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" NUMERIC_ROUNDABORT="false"></StatementSetOptions><QueryPlan DegreeOfParallelism="0" NonParallelPlanReason="NoParallelPlansInDesktopOrExpressEdition" CachedPlanSize="24" CompileTime="0" CompileCPU="0" CompileMemory="192"><MemoryGrantInfo SerialRequiredMemory="0" SerialDesiredMemory="0" GrantedMemory="0" MaxUsedMemory="0"></MemoryGrantInfo><OptimizerHardwareDependentProperties EstimatedAvailableMemoryGrant="1258238" EstimatedPagesCached="78639" EstimatedAvailableDegreeOfParallelism="1" MaxCompileMemory="1873496"></OptimizerHardwareDependentProperties><OptimizerStatsUsage><StatisticsInfo LastUpdate="2024-06-18T21:40:59.99" ModificationCount="0" SamplingPercent="100" Statistics="[PK__TableB__1788CC4C8F690C87]" Table="[TableB]" Schema="[dbo]" Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]"></StatisticsInfo><StatisticsInfo LastUpdate="2024-06-18T21:40:59.99" ModificationCount="0" SamplingPercent="100" Statistics="[PK__TableA__1788CC4C45A6C363]" Table="[TableA]" Schema="[dbo]" Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]"></StatisticsInfo></OptimizerStatsUsage><TraceFlags IsCompileTime="1"><TraceFlag Value="460" Scope="Global"></TraceFlag><TraceFlag Value="8017" Scope="Global"></TraceFlag></TraceFlags><TraceFlags IsCompileTime="0"><TraceFlag Value="460" Scope="Global"></TraceFlag><TraceFlag Value="8017" Scope="Global"></TraceFlag></TraceFlags><QueryTimeStats ElapsedTime="0" CpuTime="0"></QueryTimeStats><RelOp NodeId="0" PhysicalOp="Top" LogicalOp="Top" EstimateRows="3" EstimateIO="0" EstimateCPU="3e-07" AvgRowSize="11" EstimatedTotalSubtreeCost="0.0121842" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Column="Union1002"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="3" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><Top RowCount="0" IsPercent="0" WithTies="0"><TopExpression><ScalarOperator ScalarString="(3)"><Const ConstValue="(3)"></Const></ScalarOperator></TopExpression><RelOp NodeId="1" PhysicalOp="Stream Aggregate" LogicalOp="Aggregate" EstimateRows="3" EstimateRowsWithoutRowGoal="5.2" EstimateIO="0" EstimateCPU="6.6e-06" AvgRowSize="11" EstimatedTotalSubtreeCost="0.0121839" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Column="Union1002"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="3" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><StreamAggregate><DefinedValues></DefinedValues><GroupBy><ColumnReference Column="Union1002"></ColumnReference></GroupBy><RelOp NodeId="2" PhysicalOp="Merge Join" LogicalOp="Concatenation" EstimateRows="4.61538" EstimateRowsWithoutRowGoal="8" EstimateIO="0" EstimateCPU="0.0056176" AvgRowSize="11" EstimatedTotalSubtreeCost="0.0121801" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Column="Union1002"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="6" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualScans="0" ActualLogicalReads="0" ActualPhysicalReads="0" ActualReadAheads="0" ActualLobLogicalReads="0" ActualLobPhysicalReads="0" ActualLobReadAheads="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><Merge><DefinedValues><DefinedValue><ColumnReference Column="Union1002"></ColumnReference><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableA]" Alias="[a]" Column="UserId"></ColumnReference><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableB]" Alias="[b]" Column="UserId"></ColumnReference></DefinedValue></DefinedValues><RelOp NodeId="3" PhysicalOp="Clustered Index Scan" LogicalOp="Clustered Index Scan" EstimateRows="4" EstimatedRowsRead="4" EstimateIO="0.003125" EstimateCPU="0.0001614" AvgRowSize="11" EstimatedTotalSubtreeCost="0.0032864" TableCardinality="4" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableA]" Alias="[a]" Column="UserId"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="4" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualScans="1" ActualLogicalReads="2" ActualPhysicalReads="0" ActualReadAheads="0" ActualLobLogicalReads="0" ActualLobPhysicalReads="0" ActualLobReadAheads="0" ActualRowsRead="4" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><IndexScan Ordered="1" ScanDirection="FORWARD" ForcedIndex="0" ForceSeek="0" ForceScan="0" NoExpandHint="0" Storage="RowStore"><DefinedValues><DefinedValue><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableA]" Alias="[a]" Column="UserId"></ColumnReference></DefinedValue></DefinedValues><Object Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableA]" Index="[PK__TableA__1788CC4C45A6C363]" Alias="[a]" IndexKind="Clustered" Storage="RowStore"></Object></IndexScan></RelOp><RelOp NodeId="4" PhysicalOp="Clustered Index Scan" LogicalOp="Clustered Index Scan" EstimateRows="1" EstimateRowsWithoutRowGoal="4" EstimatedRowsRead="4" EstimateIO="0.003125" EstimateCPU="0.0001614" AvgRowSize="11" EstimatedTotalSubtreeCost="0.0032831" TableCardinality="4" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableB]" Alias="[b]" Column="UserId"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="3" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualScans="1" ActualLogicalReads="2" ActualPhysicalReads="0" ActualReadAheads="0" ActualLobLogicalReads="0" ActualLobPhysicalReads="0" ActualLobReadAheads="0" ActualRowsRead="3" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><IndexScan Ordered="1" ScanDirection="FORWARD" ForcedIndex="0" ForceSeek="0" ForceScan="0" NoExpandHint="0" Storage="RowStore"><DefinedValues><DefinedValue><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableB]" Alias="[b]" Column="UserId"></ColumnReference></DefinedValue></DefinedValues><Object Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableB]" Index="[PK__TableB__1788CC4C8F690C87]" Alias="[b]" IndexKind="Clustered" Storage="RowStore"></Object></IndexScan></RelOp></Merge></RelOp></StreamAggregate></RelOp></Top></RelOp></QueryPlan></StmtSimple></Statements></Batch></BatchSequence></ShowPlanXML>


DECLARE @previousId int = 3;

SELECT TOP (3)
  t.UserId
FROM (
    SELECT a.UserId
    FROM TableA a
    WHERE a.UserId > @previousId
    UNION ALL
    SELECT b.UserId
    FROM TableB b
    WHERE b.UserId > @previousId
) t
GROUP BY t.UserId
ORDER BY UserId;


UserId


4

5

6




Microsoft SQL Server 2005 XML Showplan


<ShowPlanXML xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan" Version="1.564" Build="16.0.1000.6"><BatchSequence><Batch><Statements><StmtSimple StatementText="SELECT TOP (3)&#xa;  t.UserId&#xa;FROM (&#xa;    SELECT a.UserId&#xa;    FROM TableA a&#xa;    WHERE a.UserId &gt; @previousId&#xa;    UNION ALL&#xa;    SELECT b.UserId&#xa;    FROM TableB b&#xa;    WHERE b.UserId &gt; @previousId&#xa;) t&#xa;GROUP BY t.UserId&#xa;ORDER BY UserId" StatementId="1" StatementCompId="2" StatementType="SELECT" StatementSqlHandle="0x0900A7416BBBC94351D0037D8788BD92096A0000000000000000000000000000000000000000000000000000" DatabaseContextSettingsId="1" ParentObjectId="0" StatementParameterizationType="0" RetrievedFromCache="true" StatementSubTreeCost="0.0121723" StatementEstRows="2.04" SecurityPolicyApplied="false" StatementOptmLevel="FULL" QueryHash="0xC178F5ADAFC958B1" QueryPlanHash="0x70855CA909C52D6F" StatementOptmEarlyAbortReason="GoodEnoughPlanFound" CardinalityEstimationModelVersion="160"><StatementSetOptions QUOTED_IDENTIFIER="true" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="true" ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" NUMERIC_ROUNDABORT="false"></StatementSetOptions><QueryPlan DegreeOfParallelism="0" NonParallelPlanReason="NoParallelPlansInDesktopOrExpressEdition" CachedPlanSize="24" CompileTime="13" CompileCPU="13" CompileMemory="232"><MemoryGrantInfo SerialRequiredMemory="0" SerialDesiredMemory="0" GrantedMemory="0" MaxUsedMemory="0"></MemoryGrantInfo><OptimizerHardwareDependentProperties EstimatedAvailableMemoryGrant="1258238" EstimatedPagesCached="78639" EstimatedAvailableDegreeOfParallelism="1" MaxCompileMemory="1873496"></OptimizerHardwareDependentProperties><OptimizerStatsUsage><StatisticsInfo LastUpdate="2024-06-18T21:40:59.99" ModificationCount="0" SamplingPercent="100" Statistics="[PK__TableB__1788CC4C8F690C87]" Table="[TableB]" Schema="[dbo]" Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]"></StatisticsInfo><StatisticsInfo LastUpdate="2024-06-18T21:40:59.99" ModificationCount="0" SamplingPercent="100" Statistics="[PK__TableA__1788CC4C45A6C363]" Table="[TableA]" Schema="[dbo]" Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]"></StatisticsInfo></OptimizerStatsUsage><TraceFlags IsCompileTime="1"><TraceFlag Value="460" Scope="Global"></TraceFlag><TraceFlag Value="8017" Scope="Global"></TraceFlag></TraceFlags><TraceFlags IsCompileTime="0"><TraceFlag Value="460" Scope="Global"></TraceFlag><TraceFlag Value="8017" Scope="Global"></TraceFlag></TraceFlags><QueryTimeStats ElapsedTime="0" CpuTime="0"></QueryTimeStats><RelOp NodeId="0" PhysicalOp="Top" LogicalOp="Top" EstimateRows="2.04" EstimateIO="0" EstimateCPU="2.04e-07" AvgRowSize="11" EstimatedTotalSubtreeCost="0.0121723" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Column="Union1002"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="3" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><Top RowCount="0" IsPercent="0" WithTies="0"><TopExpression><ScalarOperator ScalarString="(3)"><Const ConstValue="(3)"></Const></ScalarOperator></TopExpression><RelOp NodeId="1" PhysicalOp="Stream Aggregate" LogicalOp="Aggregate" EstimateRows="2.04" EstimateIO="0" EstimateCPU="2.22e-06" AvgRowSize="11" EstimatedTotalSubtreeCost="0.0121721" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Column="Union1002"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="3" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualEndOfScans="0" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><StreamAggregate><DefinedValues></DefinedValues><GroupBy><ColumnReference Column="Union1002"></ColumnReference></GroupBy><RelOp NodeId="2" PhysicalOp="Merge Join" LogicalOp="Concatenation" EstimateRows="2.4" EstimateIO="0" EstimateCPU="0.00560326" AvgRowSize="11" EstimatedTotalSubtreeCost="0.0121699" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Column="Union1002"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="3" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualScans="0" ActualLogicalReads="0" ActualPhysicalReads="0" ActualReadAheads="0" ActualLobLogicalReads="0" ActualLobPhysicalReads="0" ActualLobReadAheads="0" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><Merge><DefinedValues><DefinedValue><ColumnReference Column="Union1002"></ColumnReference><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableA]" Alias="[a]" Column="UserId"></ColumnReference><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableB]" Alias="[b]" Column="UserId"></ColumnReference></DefinedValue></DefinedValues><RelOp NodeId="3" PhysicalOp="Clustered Index Seek" LogicalOp="Clustered Index Seek" EstimateRows="1.2" EstimatedRowsRead="1.2" EstimateIO="0.003125" EstimateCPU="0.00015832" AvgRowSize="11" EstimatedTotalSubtreeCost="0.00328332" TableCardinality="4" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableA]" Alias="[a]" Column="UserId"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="1" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualScans="1" ActualLogicalReads="2" ActualPhysicalReads="0" ActualReadAheads="0" ActualLobLogicalReads="0" ActualLobPhysicalReads="0" ActualLobReadAheads="0" ActualRowsRead="1" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><IndexScan Ordered="1" ScanDirection="FORWARD" ForcedIndex="0" ForceSeek="0" ForceScan="0" NoExpandHint="0" Storage="RowStore"><DefinedValues><DefinedValue><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableA]" Alias="[a]" Column="UserId"></ColumnReference></DefinedValue></DefinedValues><Object Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableA]" Index="[PK__TableA__1788CC4C45A6C363]" Alias="[a]" IndexKind="Clustered" Storage="RowStore"></Object><SeekPredicates><SeekPredicateNew><SeekKeys><StartRange ScanType="GT"><RangeColumns><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableA]" Alias="[a]" Column="UserId"></ColumnReference></RangeColumns><RangeExpressions><ScalarOperator ScalarString="[@previousId]"><Identifier><ColumnReference Column="@previousId"></ColumnReference></Identifier></ScalarOperator></RangeExpressions></StartRange></SeekKeys></SeekPredicateNew></SeekPredicates></IndexScan></RelOp><RelOp NodeId="4" PhysicalOp="Clustered Index Seek" LogicalOp="Clustered Index Seek" EstimateRows="1.2" EstimatedRowsRead="1.2" EstimateIO="0.003125" EstimateCPU="0.00015832" AvgRowSize="11" EstimatedTotalSubtreeCost="0.00328332" TableCardinality="4" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableB]" Alias="[b]" Column="UserId"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="2" Batches="0" ActualElapsedms="0" ActualCPUms="0" ActualScans="1" ActualLogicalReads="2" ActualPhysicalReads="0" ActualReadAheads="0" ActualLobLogicalReads="0" ActualLobPhysicalReads="0" ActualLobReadAheads="0" ActualRowsRead="2" ActualEndOfScans="1" ActualExecutions="1" ActualExecutionMode="Row"></RunTimeCountersPerThread></RunTimeInformation><IndexScan Ordered="1" ScanDirection="FORWARD" ForcedIndex="0" ForceSeek="0" ForceScan="0" NoExpandHint="0" Storage="RowStore"><DefinedValues><DefinedValue><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableB]" Alias="[b]" Column="UserId"></ColumnReference></DefinedValue></DefinedValues><Object Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableB]" Index="[PK__TableB__1788CC4C8F690C87]" Alias="[b]" IndexKind="Clustered" Storage="RowStore"></Object><SeekPredicates><SeekPredicateNew><SeekKeys><StartRange ScanType="GT"><RangeColumns><ColumnReference Database="[fiddle_582761a960ef4c819bf14b900ecc9ce1]" Schema="[dbo]" Table="[TableB]" Alias="[b]" Column="UserId"></ColumnReference></RangeColumns><RangeExpressions><ScalarOperator ScalarString="[@previousId]"><Identifier><ColumnReference Column="@previousId"></ColumnReference></Identifier></ScalarOperator></RangeExpressions></StartRange></SeekKeys></SeekPredicateNew></SeekPredicates></IndexScan></RelOp></Merge></RelOp></StreamAggregate></RelOp></Top></RelOp><ParameterList><ColumnReference Column="@previousId" ParameterDataType="int" ParameterRuntimeValue="(3)"></ColumnReference></ParameterList></QueryPlan></StmtSimple></Statements></Batch></BatchSequence></ShowPlanXML>