code: func (d *ddl) AlterTableComment(ctx sessionctx.Context, ident ast.Ident, spec *ast.AlterTableSpec) error {
  	is := d.infoHandle.Get()
  	schema, ok := is.SchemaByName(ident.Schema)
  	if !ok {
  		return infoschema.ErrDatabaseNotExists.GenWithStackByArgs(ident.Schema)
  	}
  
  	tb, err := is.TableByName(ident.Schema, ident.Name)
  	if err != nil {
  		return errors.Trace(infoschema.ErrTableNotExists.GenWithStackByArgs(ident.Schema, ident.Name))
  	}
  
  	job := &model.Job{
  		SchemaID:   schema.ID,
  		TableID:    tb.Meta().ID,
  		Type:       model.ActionModifyTableComment,
  		BinlogInfo: &model.HistoryInfo{},
  		Args:       []interface{}{spec.Comment},
  	}
  
  	err = d.doDDLJob(ctx, job)
  	err = d.callHookOnChanged(err)
  	return errors.Trace(err)
  }
docs: // AlterTableComment updates the table comment information.