ddl: support specifying expr rand() as column default value

ddl/db_integration_test.go

@@ -2031,6 +2031,23 @@ func TestDefaultValueIsString(t *testing.T) {
 	require.Equal(t, "1", tbl.Meta().Columns[0].DefaultValue)
 }
 
+func TestDefaultColumnWithRand(t *testing.T) {
+	// Related issue: https://github.com/pingcap/tidb/issues/10377
+	store, clean := testkit.CreateMockStoreWithSchemaLease(t, testLease)
+	defer clean()
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+
+	tk.MustExec("drop table if exists t, t1, t2")
+	tk.MustExec("create table t (c int(10) default rand())")
+	tk.MustExec("create table t1 (c int default rand(), c1 double default rand())")
+	tk.MustExec("create table t2 (c int default rand(1), c1 double default rand(2))")
+	tk.MustExec("alter table t add column c1 int(10) default rand(2)")
+	tk.MustExec("insert into t(c) values (1),(2),(3)")
+	tk.MustExec("insert into t1(c) values (1),(2),(3)")
+	tk.MustExec("insert into t2(c) values (1),(2),(3)")
+}
+
 func TestChangingDBCharset(t *testing.T) {
 	store, clean := testkit.CreateMockStore(t)
 	defer clean()

ddl/ddl_api.go

@@ -1043,7 +1043,7 @@ func getDefaultValue(ctx sessionctx.Context, col *table.Column, c *ast.ColumnOpt
 	if tp == mysql.TypeTimestamp || tp == mysql.TypeDatetime {
 		switch x := c.Expr.(type) {
 		case *ast.FuncCallExpr:
-			if x.FnName.L == ast.CurrentTimestamp {
+			if x.FnName.L == ast.CurrentTimestamp || x.FnName.L == ast.Now {
 				defaultFsp := 0
 				if len(x.Args) == 1 {
 					if val := x.Args[0].(*driver.ValueExpr); val != nil {
@@ -1073,6 +1073,25 @@ func getDefaultValue(ctx sessionctx.Context, col *table.Column, c *ast.ColumnOpt
 
 		return value, false, nil
 	}
+
+	switch x := c.Expr.(type) {
+	case *ast.FuncCallExpr:
+		if x.FnName.L == ast.Rand {
+			if err := expression.VerifyArgsWrapper(ast.Rand, len(x.Args)); err != nil {
+				return nil, false, expression.ErrIncorrectParameterCount.GenWithStackByArgs(ast.Rand)
+			}
+			col.DefaultIsExpr = true
+			var sb strings.Builder
+			restoreFlags := format.RestoreStringSingleQuotes | format.RestoreKeyWordLowercase | format.RestoreNameBackQuotes |
+				format.RestoreSpacesAroundBinaryOperation
+			restoreCtx := format.NewRestoreCtx(restoreFlags, &sb)
+			if err := c.Expr.Restore(restoreCtx); err != nil {
+				return "", false, err
+			}
+			return sb.String(), false, nil
+		}
+	}
+
 	// handle default next value of sequence. (keep the expr string)
 	str, isSeqExpr, err := tryToGetSequenceDefaultValue(c)
 	if err != nil {