Datepart(year, …) vs. Year(…)

What are the advantages of using one over the other in the following:

DATEPART(YEAR, GETDATE())

As opposed to:

YEAR(GETDATE())

Is there is a performance difference? If so, which one is the fastest?

回答1:

Actually - using YEAR(..) is preferably for me, since it's considered a deterministic function, so if I use this in a computed column definition

ALTER TABLE dbo.MyTable
ADD YearOfDate AS YEAR(SomeDateColumn)

I can make this column persisted (and store it into the table):

ALTER TABLE dbo.MyTable
ADD YearOfDate AS YEAR(SomeDateColumn) PERSISTED

This does not work for DATEPART(YEAR, SomeDateColumn) (don't ask me why - just noticed this heuristically).

The same applies to MONTH(SomeDate) vs. DATEPART(MONTH, SomeDate).

If you have tables that you need to select from based on the month and year of a date (like SalesDate or something), then having month and years as persisted computed columns (and indexing them) can be a huge performance boost.

回答2:

There is no difference. In the execution plan both is translated to as datepart(year,getdate()).

This is true for SQL Server 2005, 2008 and 2012.

select datepart(year, getdate())
from (select 1 x) x

select year(getdate())
from (select 1 x) x

Execution plan.

<?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="1.0" Build="9.00.5057.00" xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan">
  <BatchSequence>
    <Batch>
      <Statements>
        <StmtSimple StatementCompId="1" StatementEstRows="1" StatementId="1" StatementOptmLevel="TRIVIAL" StatementSubTreeCost="1.157E-06" StatementText="select datepart(year, getdate())&#xD;&#xA;from (select 1 x) x&#xD;&#xA;&#xD;" StatementType="SELECT">
          <StatementSetOptions ANSI_NULLS="false" ANSI_PADDING="false" ANSI_WARNINGS="false" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="false" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="false" />
          <QueryPlan DegreeOfParallelism="0" CachedPlanSize="8" CompileTime="23" CompileCPU="23" CompileMemory="64">
            <RelOp AvgRowSize="11" EstimateCPU="1.157E-06" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Constant Scan" NodeId="0" Parallel="false" PhysicalOp="Constant Scan" EstimatedTotalSubtreeCost="1.157E-06">
              <OutputList>
                <ColumnReference Column="Expr1001" />
              </OutputList>
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <ConstantScan>
                <Values>
                  <Row>
                    <ScalarOperator ScalarString="datepart(year,getdate())">
                      <Identifier>
                        <ColumnReference Column="ConstExpr1002">
                          <ScalarOperator>
                            <Intrinsic FunctionName="datepart">
                              <ScalarOperator>
                                <Const ConstValue="(0)" />
                              </ScalarOperator>
                              <ScalarOperator>
                                <Intrinsic FunctionName="getdate" />
                              </ScalarOperator>
                            </Intrinsic>
                          </ScalarOperator>
                        </ColumnReference>
                      </Identifier>
                    </ScalarOperator>
                  </Row>
                </Values>
              </ConstantScan>
            </RelOp>
          </QueryPlan>
        </StmtSimple>
      </Statements>
    </Batch>
    <Batch>
      <Statements>
        <StmtSimple StatementCompId="2" StatementEstRows="1" StatementId="2" StatementOptmLevel="TRIVIAL" StatementSubTreeCost="1.157E-06" StatementText="select year(getdate())&#xD;&#xA;from (select 1 x) x" StatementType="SELECT">
          <StatementSetOptions ANSI_NULLS="false" ANSI_PADDING="false" ANSI_WARNINGS="false" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="false" NUMERIC_ROUNDABORT="false" QUOTED_IDENTIFIER="false" />
          <QueryPlan DegreeOfParallelism="0" CachedPlanSize="8" CompileTime="0" CompileCPU="0" CompileMemory="64">
            <RelOp AvgRowSize="11" EstimateCPU="1.157E-06" EstimateIO="0" EstimateRebinds="0" EstimateRewinds="0" EstimateRows="1" LogicalOp="Constant Scan" NodeId="0" Parallel="false" PhysicalOp="Constant Scan" EstimatedTotalSubtreeCost="1.157E-06">
              <OutputList>
                <ColumnReference Column="Expr1001" />
              </OutputList>
              <RunTimeInformation>
                <RunTimeCountersPerThread Thread="0" ActualRows="1" ActualEndOfScans="1" ActualExecutions="1" />
              </RunTimeInformation>
              <ConstantScan>
                <Values>
                  <Row>
                    <ScalarOperator ScalarString="datepart(year,getdate())">
                      <Identifier>
                        <ColumnReference Column="ConstExpr1002">
                          <ScalarOperator>
                            <Intrinsic FunctionName="datepart">
                              <ScalarOperator>
                                <Const ConstValue="(0)" />
                              </ScalarOperator>
                              <ScalarOperator>
                                <Intrinsic FunctionName="getdate" />
                              </ScalarOperator>
                            </Intrinsic>
                          </ScalarOperator>
                        </ColumnReference>
                      </Identifier>
                    </ScalarOperator>
                  </Row>
                </Values>
              </ConstantScan>
            </RelOp>
          </QueryPlan>
        </StmtSimple>
      </Statements>
    </Batch>
  </BatchSequence>
</ShowPlanXML>