From ec849d1491d3fbee3ae11b3ecf7777ff41f87f09 Mon Sep 17 00:00:00 2001
From: "TONG, Zhigao" <solotzg@163.com>
Date: Tue, 14 May 2024 17:09:18 +0800
Subject: [PATCH] Revert "expression: unify casting real to string in tidb
 (tikv#16975) (#53129)"

This reverts commit f311d7751aa0a7834addbea6ddbbe63f56382461.
---
 pkg/expression/builtin_cast.go      | 36 +------------------
 pkg/expression/builtin_cast_test.go | 55 -----------------------------
 pkg/expression/builtin_cast_vec.go  |  2 +-
 3 files changed, 2 insertions(+), 91 deletions(-)

diff --git a/pkg/expression/builtin_cast.go b/pkg/expression/builtin_cast.go
index 3aac238d86e10..134b6b88ce0a5 100644
--- a/pkg/expression/builtin_cast.go
+++ b/pkg/expression/builtin_cast.go
@@ -23,7 +23,6 @@
 package expression
 
 import (
-	"bytes"
 	"fmt"
 	"math"
 	"strconv"
@@ -1037,39 +1036,6 @@ func (b *builtinCastRealAsStringSig) Clone() builtinFunc {
 	return newSig
 }
 
-func formatFloat(f float64, bitSize int) string {
-	// MySQL makes float to string max 6 significant digits
-	// MySQL makes double to string max 17 significant digits
-
-	// Unified to display behavior of TiDB. TiKV should also follow this rule.
-	const (
-		expFormatBig   = 1e15
-		expFormatSmall = 1e-15
-	)
-
-	absVal := math.Abs(f)
-	isEFormat := false
-
-	if bitSize == 32 {
-		isEFormat = float32(absVal) >= float32(expFormatBig) || (float32(absVal) != 0 && float32(absVal) < float32(expFormatSmall))
-	} else {
-		isEFormat = absVal >= expFormatBig || (absVal != 0 && absVal < expFormatSmall)
-	}
-
-	dst := make([]byte, 0, 24)
-	if isEFormat {
-		dst = strconv.AppendFloat(dst, f, 'e', -1, bitSize)
-		if idx := bytes.IndexByte(dst, '+'); idx != -1 {
-			copy(dst[idx:], dst[idx+1:])
-			dst = dst[:len(dst)-1]
-		}
-	} else {
-		dst = strconv.AppendFloat(dst, f, 'f', -1, bitSize)
-	}
-
-	return string(dst)
-}
-
 func (b *builtinCastRealAsStringSig) evalString(ctx EvalContext, row chunk.Row) (res string, isNull bool, err error) {
 	val, isNull, err := b.args[0].EvalReal(ctx, row)
 	if isNull || err != nil {
@@ -1083,7 +1049,7 @@ func (b *builtinCastRealAsStringSig) evalString(ctx EvalContext, row chunk.Row)
 		// If we strconv.FormatFloat the value with 64bits, the result is incorrect!
 		bits = 32
 	}
-	res, err = types.ProduceStrWithSpecifiedTp(formatFloat(val, bits), b.tp, typeCtx(ctx), false)
+	res, err = types.ProduceStrWithSpecifiedTp(strconv.FormatFloat(val, 'f', -1, bits), b.tp, typeCtx(ctx), false)
 	if err != nil {
 		return res, false, err
 	}
diff --git a/pkg/expression/builtin_cast_test.go b/pkg/expression/builtin_cast_test.go
index 4e60397a199b8..4f106a5837961 100644
--- a/pkg/expression/builtin_cast_test.go
+++ b/pkg/expression/builtin_cast_test.go
@@ -1116,61 +1116,6 @@ func TestCastFuncSig(t *testing.T) {
 	require.False(t, iRes.IsNull())
 	require.Equal(t, types.KindInt64, iRes.Kind())
 	require.Equal(t, int64(0), iRes.GetInt64())
-
-	// test cast real to string
-	zero := float32(0.00)
-	negZero := -zero
-	castFloat32ToStringCases := []struct {
-		v      float32
-		expect string
-	}{
-		{negZero, "-0"},
-		{00000.0, "0"},
-		{4474.78125, "4474.7812"},
-		{1e15, "1e15"},
-		{-1e15, "-1e15"},
-		{9.99999e14, "999999000000000"},
-		{-9.99999e14, "-999999000000000"},
-		{1e15 - 1.0, "1e15"},
-		{-3.4028235e38, "-3.4028235e38"},
-		{3.4028235e38, "3.4028235e38"},
-		{1.1754944e-38, "1.1754944e-38"},
-		{1.000, "1"},
-		{-123456789123000.0, "-123456790000000"},
-		{1e-15, "0.000000000000001"},
-		{9.9999e-16, "9.9999e-16"},
-		{1.23456789123000e-9, "0.0000000012345679"},
-	}
-
-	for _, d := range castFloat32ToStringCases {
-		require.Equal(t, formatFloat(float64(d.v), 32), d.expect)
-	}
-
-	castFloat64ToStringCases := []struct {
-		v      float64
-		expect string
-	}{
-		{float64(negZero), "-0"},
-		{00000.0, "0"},
-		{4474.78125, "4474.78125"},
-		{1e15, "1e15"},
-		{-1e15, "-1e15"},
-		{9.99999e14, "999999000000000"},
-		{-9.99999e14, "-999999000000000"},
-		{1e15 - 1.0, "999999999999999"},
-		{-1.7976931348623157e308, "-1.7976931348623157e308"},
-		{1.7976931348623157e308, "1.7976931348623157e308"},
-		{2.2250738585072014e-308, "2.2250738585072014e-308"},
-		{1.000, "1"},
-		{-123456789123000.0, "-123456789123000"},
-		{1e-15, "0.000000000000001"},
-		{9.9999e-16, "9.9999e-16"},
-		{1.23456789123000e-9, "0.00000000123456789123"},
-	}
-
-	for _, d := range castFloat64ToStringCases {
-		require.Equal(t, formatFloat(d.v, 64), d.expect)
-	}
 }
 
 func TestCastJSONAsDecimalSig(t *testing.T) {
diff --git a/pkg/expression/builtin_cast_vec.go b/pkg/expression/builtin_cast_vec.go
index 0ffe672b7f0e1..bce2383171338 100644
--- a/pkg/expression/builtin_cast_vec.go
+++ b/pkg/expression/builtin_cast_vec.go
@@ -221,7 +221,7 @@ func (b *builtinCastRealAsStringSig) vecEvalString(ctx EvalContext, input *chunk
 			result.AppendNull()
 			continue
 		}
-		res, err = types.ProduceStrWithSpecifiedTp(formatFloat(v, bits), b.tp, tc, false)
+		res, err = types.ProduceStrWithSpecifiedTp(strconv.FormatFloat(v, 'f', -1, bits), b.tp, tc, false)
 		if err != nil {
 			return err
 		}