executor: refine the precision for avg

executor/aggfuncs/func_avg.go

@@ -14,6 +14,8 @@
 package aggfuncs
 
 import (
+	"github.com/cznic/mathutil"
+	"github.com/pingcap/tidb/mysql"
 	"github.com/pingcap/tidb/sessionctx"
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/util/chunk"
@@ -56,7 +58,19 @@ func (e *baseAvgDecimal) AppendFinalResult2Chunk(sctx sessionctx.Context, pr Par
 	finalResult := new(types.MyDecimal)
 	err := types.DecimalDiv(&p.sum, decimalCount, finalResult, types.DivFracIncr)
 	if err != nil {
-		return errors.Trace(err)
+		return err
+	}
+	// Make the decimal be the result of type inferring.
+	frac := e.args[0].GetType().Decimal
+	if len(e.args) == 2 {
+		frac = e.args[1].GetType().Decimal
+	}
+	if frac == -1 {
+		frac = mysql.MaxDecimalScale
+	}
+	err = finalResult.Round(finalResult, mathutil.Min(frac, mysql.MaxDecimalScale), types.ModeHalfEven)
+	if err != nil {
+		return err
 	}
 	chk.AppendMyDecimal(e.ordinal, finalResult)
 	return nil
@@ -195,6 +209,15 @@ func (e *avgOriginal4DistinctDecimal) AppendFinalResult2Chunk(sctx sessionctx.Co
 	if err != nil {
 		return errors.Trace(err)
 	}
+	// Make the decimal be the result of type inferring.
+	frac := e.args[0].GetType().Decimal
+	if frac == -1 {
+		frac = mysql.MaxDecimalScale
+	}
+	err = finalResult.Round(finalResult, mathutil.Min(frac, mysql.MaxDecimalScale), types.ModeHalfEven)
+	if err != nil {
+		return err
+	}
 	chk.AppendMyDecimal(e.ordinal, finalResult)
 	return nil
 }

executor/builder.go

@@ -887,6 +887,27 @@ func (b *executorBuilder) wrapCastForAggArgs(funcs []*aggregation.AggFuncDesc) {
 		}
 		for i := range f.Args {
 			f.Args[i] = castFunc(b.ctx, f.Args[i])
+			if f.Name != ast.AggFuncAvg && f.Name != ast.AggFuncSum {
+				continue
+			}
+			// After wrapping cast on the argument, flen etc. may not the same
+			// as the type of the aggregation function. The following part set
+			// the type of the argument exactly as the type of the aggregation
+			// function.
+			// Note: If the `Tp` of argument is the same as the `Tp` of the
+			// aggregation function, it will not wrap cast function on it
+			// internally. The reason of the special handling for `Column` is
+			// that the `RetType` of `Column` refers to the `infoschema`, so we
+			// need to set a new variable for it to avoid modifying the
+			// definition in `infoschema`.
+			if col, ok := f.Args[i].(*expression.Column); ok {
+				col.RetType = types.NewFieldType(col.RetType.Tp)
+			}
+			// originTp is used when the the `Tp` of column is TypeFloat32 while
+			// the type of the aggregation function is TypeFloat64.
+			originTp := f.Args[i].GetType().Tp
+			*(f.Args[i].GetType()) = *(f.RetTp)
+			f.Args[i].GetType().Tp = originTp
 		}
 	}
 }

expression/aggregation/descriptor.go

@@ -291,21 +291,21 @@ func (a *AggFuncDesc) typeInfer4Count(ctx sessionctx.Context) {
 // Because child returns integer or decimal type.
 func (a *AggFuncDesc) typeInfer4Sum(ctx sessionctx.Context) {
 	switch a.Args[0].GetType().Tp {
-	case mysql.TypeTiny, mysql.TypeShort, mysql.TypeInt24, mysql.TypeLong, mysql.TypeLonglong, mysql.TypeNewDecimal:
+	case mysql.TypeTiny, mysql.TypeShort, mysql.TypeInt24, mysql.TypeLong, mysql.TypeLonglong:
+		a.RetTp = types.NewFieldType(mysql.TypeNewDecimal)
+		a.RetTp.Flen, a.RetTp.Decimal = mysql.MaxDecimalWidth, 0
+	case mysql.TypeNewDecimal:
 		a.RetTp = types.NewFieldType(mysql.TypeNewDecimal)
 		a.RetTp.Flen, a.RetTp.Decimal = mysql.MaxDecimalWidth, a.Args[0].GetType().Decimal
 		if a.RetTp.Decimal < 0 || a.RetTp.Decimal > mysql.MaxDecimalScale {
 			a.RetTp.Decimal = mysql.MaxDecimalScale
 		}
-		// TODO: a.Args[0] = expression.WrapWithCastAsDecimal(ctx, a.Args[0])
 	case mysql.TypeDouble, mysql.TypeFloat:
 		a.RetTp = types.NewFieldType(mysql.TypeDouble)
 		a.RetTp.Flen, a.RetTp.Decimal = mysql.MaxRealWidth, a.Args[0].GetType().Decimal
-		//TODO: a.Args[0] = expression.WrapWithCastAsReal(ctx, a.Args[0])
 	default:
 		a.RetTp = types.NewFieldType(mysql.TypeDouble)
 		a.RetTp.Flen, a.RetTp.Decimal = mysql.MaxRealWidth, types.UnspecifiedLength
-		// TODO: a.Args[0] = expression.WrapWithCastAsReal(ctx, a.Args[0])
 	}
 	types.SetBinChsClnFlag(a.RetTp)
 }
@@ -322,15 +322,12 @@ func (a *AggFuncDesc) typeInfer4Avg(ctx sessionctx.Context) {
 			a.RetTp.Decimal = mathutil.Min(a.Args[0].GetType().Decimal+types.DivFracIncr, mysql.MaxDecimalScale)
 		}
 		a.RetTp.Flen = mysql.MaxDecimalWidth
-		// TODO: a.Args[0] = expression.WrapWithCastAsDecimal(ctx, a.Args[0])
 	case mysql.TypeDouble, mysql.TypeFloat:
 		a.RetTp = types.NewFieldType(mysql.TypeDouble)
 		a.RetTp.Flen, a.RetTp.Decimal = mysql.MaxRealWidth, a.Args[0].GetType().Decimal
-		// TODO: a.Args[0] = expression.WrapWithCastAsReal(ctx, a.Args[0])
 	default:
 		a.RetTp = types.NewFieldType(mysql.TypeDouble)
 		a.RetTp.Flen, a.RetTp.Decimal = mysql.MaxRealWidth, types.UnspecifiedLength
-		// TODO: a.Args[0] = expression.WrapWithCastAsReal(ctx, a.Args[0])
 	}
 	types.SetBinChsClnFlag(a.RetTp)
 }