func floatOrDecimalBuiltin1(f func(float64) (Datum, error)) []builtin {
	return []builtin{
		{
			types:      argTypes{floatType},
			returnType: typeFloat,
			fn: func(_ EvalContext, args DTuple) (Datum, error) {
				return f(float64(args[0].(DFloat)))
			},
		}, {
			types:      argTypes{decimalType},
			returnType: typeDecimal,
			fn: func(_ EvalContext, args DTuple) (Datum, error) {
				dec := args[0].(*DDecimal)
				v, err := decimal.Float64FromDec(&dec.Dec)
				if err != nil {
					return nil, err
				}
				r, err := f(v)
				if err != nil {
					return r, err
				}
				rf := float64(r.(DFloat))
				if math.IsNaN(rf) || math.IsInf(rf, 0) {
					// TODO(nvanbenschoten) NaN semmantics should be introduced
					// into the decimal library to support it here.
					return nil, fmt.Errorf("decimal does not support NaN")
				}
				dd := &DDecimal{}
				decimal.SetFromFloat(&dd.Dec, rf)
				return dd, nil
			},
		},
	}
}

func ddecimal(f float64) copyableExpr {
	return func() Expr {
		dd := &DDecimal{}
		decimal.SetFromFloat(&dd.Dec, f)
		return dd
	}
}

func makeDecimalTestDatum(count int) []parser.Datum {
	rng, _ := randutil.NewPseudoRand()

	vals := make([]parser.Datum, count)
	for i := range vals {
		dd := &parser.DDecimal{}
		decimal.SetFromFloat(&dd.Dec, rng.Float64())
		vals[i] = dd
	}
	return vals
}

//.........这里部分代码省略.........
				return DFloat(1), nil
			}
			return DFloat(0), nil
		case DInt:
			return DFloat(v), nil
		case DFloat:
			return d, nil
		case *DDecimal:
			f, err := decimal.Float64FromDec(&v.Dec)
			if err != nil {
				return nil, errFloatOutOfRange
			}
			return DFloat(f), nil
		case DString:
			f, err := strconv.ParseFloat(string(v), 64)
			if err != nil {
				return nil, err
			}
			return DFloat(f), nil
		}

	case *DecimalType:
		dd := &DDecimal{}
		switch v := d.(type) {
		case DBool:
			if v {
				dd.SetUnscaled(1)
			}
			return dd, nil
		case DInt:
			dd.SetUnscaled(int64(v))
			return dd, nil
		case DFloat:
			decimal.SetFromFloat(&dd.Dec, float64(v))
			return dd, nil
		case *DDecimal:
			return d, nil
		case DString:
			if _, ok := dd.SetString(string(v)); !ok {
				return nil, fmt.Errorf("could not parse string %q as decimal", v)
			}
			return dd, nil
		}

	case *StringType:
		var s DString
		switch t := d.(type) {
		case DBool, DInt, DFloat, *DDecimal, dNull:
			s = DString(d.String())
		case DString:
			s = t
		case DBytes:
			if !utf8.ValidString(string(t)) {
				return nil, fmt.Errorf("invalid utf8: %q", string(t))
			}
			s = DString(t)
		}
		if c, ok := expr.Type.(*StringType); ok {
			// If the CHAR type specifies a limit we truncate to that limit:
			//   'hello'::CHAR(2) -> 'he'
			if c.N > 0 && c.N < len(s) {
				s = s[:c.N]
			}
		}
		return s, nil

func TestNumericConstantAvailableTypes(t *testing.T) {
	wantInt := numValAvailIntFloatDec
	wantFloatButCanBeInt := numValAvailFloatIntDec
	wantFloat := numValAvailFloatDec

	testCases := []struct {
		str   string
		avail []Datum
	}{
		{"1", wantInt},
		{"0", wantInt},
		{"-1", wantInt},
		{"9223372036854775807", wantInt},
		{"1.0", wantFloatButCanBeInt},
		{"-1234.0000", wantFloatButCanBeInt},
		{"1e10", wantFloatButCanBeInt},
		{"1E10", wantFloatButCanBeInt},
		{"1.1", wantFloat},
		{"1e-10", wantFloat},
		{"1E-10", wantFloat},
		{"-1231.131", wantFloat},
		{"876543234567898765436787654321", wantFloat},
	}

	for i, test := range testCases {
		tok := token.INT
		if strings.ContainsAny(test.str, ".eE") {
			tok = token.FLOAT
		}
		val := constant.MakeFromLiteral(test.str, tok, 0)
		if val.Kind() == constant.Unknown {
			t.Fatalf("%d: could not parse value string %q", i, test.str)
		}

		// Check available types.
		c := &NumVal{Value: val}
		avail := c.AvailableTypes()
		if !reflect.DeepEqual(avail, test.avail) {
			t.Errorf("%d: expected the available type set %v for %v, found %v",
				i, test.avail, c.Value.ExactString(), avail)
		}

		// Make sure it can be resolved as each of those types.
		for _, availType := range avail {
			if res, err := c.ResolveAsType(availType); err != nil {
				t.Errorf("%d: expected resolving %v as available type %s would succeed, found %v",
					i, c.Value.ExactString(), availType.Type(), err)
			} else {
				resErr := func(parsed, resolved interface{}) {
					t.Errorf("%d: expected resolving %v as available type %s would produce a Datum with the value %v, found %v",
						i, c, availType.Type(), parsed, resolved)
				}
				switch typ := res.(type) {
				case *DInt:
					var i int64
					var err error
					if tok == token.INT {
						if i, err = strconv.ParseInt(test.str, 10, 64); err != nil {
							t.Fatal(err)
						}
					} else {
						var f float64
						if f, err = strconv.ParseFloat(test.str, 64); err != nil {
							t.Fatal(err)
						}
						i = int64(f)
					}
					if resI := int64(*typ); i != resI {
						resErr(i, resI)
					}
				case *DFloat:
					f, err := strconv.ParseFloat(test.str, 64)
					if err != nil {
						t.Fatal(err)
					}
					if resF := float64(*typ); f != resF {
						resErr(f, resF)
					}
				case *DDecimal:
					d := new(inf.Dec)
					if !strings.ContainsAny(test.str, "eE") {
						if _, ok := d.SetString(test.str); !ok {
							t.Fatal(fmt.Sprintf("could not set %q on decimal", test.str))
						}
					} else {
						f, err := strconv.ParseFloat(test.str, 64)
						if err != nil {
							t.Fatal(err)
						}
						decimal.SetFromFloat(d, f)
					}
					if resD := &typ.Dec; d.Cmp(resD) != 0 {
						resErr(d, resD)
					}
				}
			}
		}
	}
}