executor:adjust 'show create table' for mysql compatibility

[xiekeyi98]

show create table should not print the charset of the column
if it is the same with table charset( same with MySQL)

What problem does this PR solve?

If the charset of column is the same with table charset , we should better not show the carset of the column.
(In order to be consistent with MySQL.)

What is changed and how it works?

Change func (e *ShowExec) fetchShowCreateTable() error{} , to determine if the column charset and table charset are equal.

Check List

Tests

• Unit test

Before

mysql> show create table ptest\G
*************************** 1. row ***************************
       Table: ptest
Create Table: CREATE TABLE `ptest` (
  `a` int(11) NOT NULL,
  `b` double NOT NULL DEFAULT '2.0',
  `c` varchar(10) CHARSET utf8mb4 COLLATE utf8mb4_bin NOT NULL,
  `d` time DEFAULT NULL,
  `e` timestamp NULL DEFAULT NULL,
  `f` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`a`),
  UNIQUE KEY `d` (`d`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
1 row in set (0.00 sec)

After

mysql> show create table ptest\G
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    1
Current database: test

*************************** 1. row ***************************
       Table: ptest
Create Table: CREATE TABLE `ptest` (
  `a` int(11) NOT NULL,
  `b` double NOT NULL DEFAULT '2.0',
  `c` varchar(10) NOT NULL,
  `d` time DEFAULT NULL,
  `e` timestamp NULL DEFAULT NULL,
  `f` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`a`),
  UNIQUE KEY `d` (`d`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
1 row in set (0.00 sec)

[codecov-io]

Codecov Report

Merging #9113 into master will increase coverage by <.01%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##           master    #9113      +/-   ##
==========================================
+ Coverage   67.17%   67.18%   +<.01%     
==========================================
  Files         371      371              
  Lines       77524    77525       +1     
==========================================
+ Hits        52079    52083       +4     
+ Misses      20791    20789       -2     
+ Partials     4654     4653       -1

Impacted Files	Coverage Δ
executor/show.go	43.26% <45.45%> (+0.08%)	⬆️
executor/join.go	77.86% <0%> (-0.53%)	⬇️
executor/distsql.go	73% <0%> (+0.46%)	⬆️
util/systimemon/systime_mon.go	100% <0%> (+20%)	⬆️

---

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[xiekeyi98]

/run-all-tests

[xiekeyi98]

/run-common-test tidb-test=pr/724
/run-integration-common-test tidb-test=pr/724

[xiekeyi98]

/run-common-test tidb-test=pr/724
/run-integration-common-test tidb-test=pr/724

[xiekeyi98]

/run-common-test tidb-test=pr/724
/run-integration-common-test tidb-test=pr/724

[xiekeyi98]

/run-all-tests tidb-test=pr/724

[xiekeyi98]

/run-all-tests tidb-test=pr/724

[xiekeyi98]

/run-all-tests tidb-test=pr/724

[xiekeyi98]

/run-all-tests tidb-test=pr/724

[xiekeyi98]

/run-common-test tidb-test=pr/724
/run-integration-common-test tidb-test=pr/724

[xiekeyi98]

/run-common-test tidb-test=pr/724

[AndrewDi]

@xiekeyi98 For your reference in my test, if we create column with specific charset, even column's charset is the same as table's charset, SHOW CREATE TABLE will show CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci part.

mysql> use test
Database changed
mysql> create table t (a int,b varchar(20));
Query OK, 0 rows affected (0.02 sec)

mysql> show create table t\G
*************************** 1. row ***************************
       Table: t
Create Table: CREATE TABLE `t` (
  `a` int(11) DEFAULT NULL,
  `b` varchar(20) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

mysql> create table t1 (a int,b varchar(20) charset utf8mb4);
Query OK, 0 rows affected (0.02 sec)

mysql> show create table t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `a` int(11) DEFAULT NULL,
  `b` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

mysql> select version()\G
*************************** 1. row ***************************
version(): 8.0.15
1 row in set (0.00 sec)

[xiekeyi98]

@AndrewDi Thanks for your help.
Would you mind review my code again?

[xiekeyi98]

Because I don't understand how MYSQL handles it, I rolled back some commits.
This PR will only resolve table && column are inconsistent.
I will create new issues to explain more.

[xiekeyi98]

/run-all-tests tidb-test=pr/724

[xiekeyi98]

/run-all-tests tidb-test=pr/724

[crazycs520]

LGTM

[winkyao]

LGTM, please cherry-pick this PR to 2.1 branch