ddl: fix double/float data not being truncated for column modificatio…

…n operations when the number of decimal places is reduced. (#41555) close #41281
pingcap · Feb 20, 2023 · 109b3b6 · 109b3b6
1 parent df249e9
commit 109b3b6
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Showing 2 changed files with 7 additions and 2 deletions.
diff --git a/ddl/column.go b/ddl/column.go
@@ -364,7 +364,8 @@ func needChangeColumnData(oldCol, newCol *model.ColumnInfo) bool {
 	toUnsigned := mysql.HasUnsignedFlag(newCol.GetFlag())
 	originUnsigned := mysql.HasUnsignedFlag(oldCol.GetFlag())
 	needTruncationOrToggleSign := func() bool {
-		return (newCol.GetFlen() > 0 && newCol.GetFlen() < oldCol.GetFlen()) || (toUnsigned != originUnsigned)
+		return (newCol.GetFlen() > 0 && (newCol.GetFlen() < oldCol.GetFlen() || newCol.GetDecimal() < oldCol.GetDecimal())) ||
+			(toUnsigned != originUnsigned)
 	}
 	// Ignore the potential max display length represented by integer's flen, use default flen instead.
 	defaultOldColFlen, _ := mysql.GetDefaultFieldLengthAndDecimal(oldCol.GetType())

diff --git a/ddl/column_type_change_test.go b/ddl/column_type_change_test.go
@@ -2321,11 +2321,15 @@ func TestColumnTypeChangeBetweenFloatAndDouble(t *testing.T) {
 	prepare := func(createTableStmt string) {
 		tk.MustExec("drop table if exists t;")
 		tk.MustExec(createTableStmt)
-		tk.MustExec("insert into t values (36.4), (24.1);")
+		tk.MustExec("insert into t values (36.43), (24.1);")
 	}
 
 	prepare("create table t (a float(6,2));")
 	tk.MustExec("alter table t modify a double(6,2)")
+	tk.MustQuery("select a from t;").Check(testkit.Rows("36.43", "24.1"))
+
+	prepare("create table t (a float(6,2));")
+	tk.MustExec("alter table t modify a float(6,1)")
 	tk.MustQuery("select a from t;").Check(testkit.Rows("36.4", "24.1"))
 
 	prepare("create table t (a double(6,2));")