Split ALTER related data type specific code in sql_table.cc to Type_handler

    if ((def->real_field_type() == MYSQL_TYPE_DATE ||

         def->real_field_type() == MYSQL_TYPE_NEWDATE ||

         def->real_field_type() == MYSQL_TYPE_DATETIME ||

         def->real_field_type() == MYSQL_TYPE_DATETIME2) &&

         !alter_ctx->datetime_field &&

         !(~def->flags & (NO_DEFAULT_VALUE_FLAG | NOT_NULL_FLAG)) &&

         thd->variables.sql_mode & MODE_NO_ZERO_DATE)

    {

        alter_ctx->datetime_field= def;

        alter_ctx->error_if_not_empty= TRUE;

    }

    switch (alter_ctx.datetime_field->real_field_type())

    {

      case MYSQL_TYPE_DATE:

      case MYSQL_TYPE_NEWDATE:

        f_val= "0000-00-00";

        t_type= MYSQL_TIMESTAMP_DATE;

        break;

      case MYSQL_TYPE_DATETIME:

      case MYSQL_TYPE_DATETIME2:

        f_val= "0000-00-00 00:00:00";

        t_type= MYSQL_TIMESTAMP_DATETIME;

        break;

      default:

        /* Shouldn't get here. */

        DBUG_ASSERT(0);

    }

virtual const Name default_value() const;

virtual bool validate_implicit_default_value(THD *thd,

                                             const Column_definition &def)

                                             const;