decimal type in show create table is not same as MySQL

[imtbkcat]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

      create table decimalschema(x decimal(15));
      show create table decimalschema;
   

2. What did you expect to see?

    +---------------+-------------------------------------------------------------------------------- 
   ----------------------------------------------+
    | Table         | Create Table                                                                                                                 |
    +---------------+-------------------------------------------------------------------------------- 
   ----------------------------------------------+
    | decimalschema | CREATE TABLE `decimalschema` (
      `x` decimal(15,0) DEFAULT NULL
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
    +---------------+-------------------------------------------------------------------------------- 
   ----------------------------------------------+
    1 row in set (0.00 sec)
   

3. What did you see instead?

    +---------------+-------------------------------------------------------------------------------- 
   ----------------------------------------------+
    | Table         | Create Table                                                                                                                 |
    +---------------+-------------------------------------------------------------------------------- 
   ----------------------------------------------+
    | decimalschema | CREATE TABLE `decimalschema` (
      `x` decimal(15) DEFAULT NULL
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
    +---------------+-------------------------------------------------------------------------------- 
   ----------------------------------------------+
    1 row in set (0.00 sec)
   

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   v2.1.0