From fa952307bfc8a0352f385c0abae9803aafeefc8d Mon Sep 17 00:00:00 2001
From: Rain Li <blacktear23@gmail.com>
Date: Tue, 29 Dec 2020 14:35:13 +0800
Subject: [PATCH] ddl: fix default decimal Flen value (#22036)

---
 ddl/db_test.go             | 35 +++++++++++++++++++++++++++++++++++
 planner/core/preprocess.go |  6 ++++++
 2 files changed, 41 insertions(+)

diff --git a/ddl/db_test.go b/ddl/db_test.go
index 6d7fab25f5..5a06cbaa61 100644
--- a/ddl/db_test.go
+++ b/ddl/db_test.go
@@ -6489,3 +6489,38 @@ func (s *testDBSuite4) TestUnsupportedAlterTableOption(c *C) {
 	tk.MustExec("create table t(a char(10) not null,b char(20)) shard_row_id_bits=6;")
 	tk.MustGetErrCode("alter table t pre_split_regions=6;", errno.ErrUnsupportedDDLOperation)
 }
+
+func (s *testDBSuite4) TestCreateTableWithDecimalWithDoubleZero(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+
+	checkType := func(db, table, field string) {
+		ctx := tk.Se.(sessionctx.Context)
+		is := domain.GetDomain(ctx).InfoSchema()
+		tableInfo, err := is.TableByName(model.NewCIStr(db), model.NewCIStr(table))
+		c.Assert(err, IsNil)
+		tblInfo := tableInfo.Meta()
+		for _, col := range tblInfo.Columns {
+			if col.Name.L == field {
+				c.Assert(col.Flen, Equals, 10)
+			}
+		}
+	}
+
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists tt")
+	tk.MustExec("create table tt(d decimal(0, 0))")
+	checkType("test", "tt", "d")
+
+	tk.MustExec("drop table tt")
+	tk.MustExec("create table tt(a int)")
+	tk.MustExec("alter table tt add column d decimal(0, 0)")
+	checkType("test", "tt", "d")
+
+	/*
+		Currently not support change column to decimal
+		tk.MustExec("drop table tt")
+		tk.MustExec("create table tt(d int)")
+		tk.MustExec("alter table tt change column d d decimal(0, 0)")
+		checkType("test", "tt", "d")
+	*/
+}
diff --git a/planner/core/preprocess.go b/planner/core/preprocess.go
index 68e9e28fe3..5fd78a2d59 100644
--- a/planner/core/preprocess.go
+++ b/planner/core/preprocess.go
@@ -992,6 +992,12 @@ func checkColumn(colDef *ast.ColumnDef) error {
 		if tp.Flen > mysql.MaxDecimalWidth {
 			return types.ErrTooBigPrecision.GenWithStackByArgs(tp.Flen, colDef.Name.Name.O, mysql.MaxDecimalWidth)
 		}
+
+		// If decimal and flen all equals 0, just set flen to default value.
+		if tp.Decimal == 0 && tp.Flen == 0 {
+			defaultFlen, _ := mysql.GetDefaultFieldLengthAndDecimal(mysql.TypeNewDecimal)
+			tp.Flen = defaultFlen
+		}
 	case mysql.TypeBit:
 		if tp.Flen <= 0 {
 			return types.ErrInvalidFieldSize.GenWithStackByArgs(colDef.Name.Name.O)