From d97b8ed67825a2ea0e20657e546c6c187a0afb1d Mon Sep 17 00:00:00 2001
From: tiancaiamao <tiancaiamao@gmail.com>
Date: Thu, 31 Jan 2019 14:54:23 +0800
Subject: [PATCH] expression: fix precision when casting float to string
 (#9137) (#9227)

---
 expression/builtin_cast.go     | 15 +++++++++++++--
 expression/integration_test.go |  6 ++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/expression/builtin_cast.go b/expression/builtin_cast.go
index f623899032f41..5731e2abee065 100644
--- a/expression/builtin_cast.go
+++ b/expression/builtin_cast.go
@@ -796,9 +796,20 @@ func (b *builtinCastRealAsStringSig) Clone() builtinFunc {
 func (b *builtinCastRealAsStringSig) evalString(row chunk.Row) (res string, isNull bool, err error) {
 	val, isNull, err := b.args[0].EvalReal(b.ctx, row)
 	if isNull || err != nil {
-		return res, isNull, errors.Trace(err)
+		return res, isNull, err
+	}
+
+	bits := 64
+	if b.args[0].GetType().Tp == mysql.TypeFloat {
+		// b.args[0].EvalReal() casts the value from float32 to float64, for example:
+		// float32(208.867) is cast to float64(208.86700439)
+		// If we strconv.FormatFloat the value with 64bits, the result is incorrect!
+		bits = 32
+	}
+	res, err = types.ProduceStrWithSpecifiedTp(strconv.FormatFloat(val, 'f', -1, bits), b.tp, b.ctx.GetSessionVars().StmtCtx)
+	if err != nil {
+		return res, false, err
 	}
-	res, err = types.ProduceStrWithSpecifiedTp(strconv.FormatFloat(val, 'f', -1, 64), b.tp, b.ctx.GetSessionVars().StmtCtx)
 	return res, isNull, errors.Trace(err)
 }
 
diff --git a/expression/integration_test.go b/expression/integration_test.go
index 8c7922ae36a38..c01f8364a24ec 100644
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -572,6 +572,12 @@ func (s *testIntegrationSuite) TestStringBuiltin(c *C) {
 	result.Check(testkit.Rows("<nil>"))
 	result = tk.MustQuery("select concat(null, a, b) from t")
 	result.Check(testkit.Rows("<nil>"))
+	tk.MustExec("drop table if exists t")
+	// Fix issue 9123
+	tk.MustExec("create table t(a char(32) not null, b float default '0') engine=innodb default charset=utf8mb4")
+	tk.MustExec("insert into t value('0a6f9d012f98467f8e671e9870044528', 208.867)")
+	result = tk.MustQuery("select concat_ws( ',', b) from t where a = '0a6f9d012f98467f8e671e9870044528';")
+	result.Check(testkit.Rows("208.867"))
 
 	// for concat_ws
 	tk.MustExec("drop table if exists t")