ddl: modify different character sets varchar type column maximum length limit

[ciscoxll]

What problem does this PR solve?

We execute the following sql , found that tidb and mysql behave differently.

# tidb master branch / tidb v2.1.0:

tidb>  CREATE TABLE tr (id INT, name VARCHAR(20000), purchased DATE )  DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
ERROR 1074 (42000): Column length too big for column 'name' (max = 16383); use BLOB or TEXT instead

# tidb v2.0.0 :

MySQL [test]> CREATE TABLE tr (id INT, name VARCHAR(20000), purchased DATE )  DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
Query OK, 0 rows affected (0.01 sec)

mysql>  CREATE TABLE tr (id INT, name VARCHAR(65536), purchased DATE )  DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
ERROR 1074 (42000): Column length too big for column 'name' (max = 21845); use BLOB or TEXT instead

# mysql 5.7
mysql>  CREATE TABLE tr (id INT, name VARCHAR(65536), purchased DATE )  DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
ERROR 1074 (42000): Column length too big for column 'name' (max = 16383); use BLOB or TEXT instead

mysql>  CREATE TABLE tr (id INT, name VARCHAR(65536), purchased DATE )  DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
ERROR 1074 (42000): Column length too big for column 'name' (max = 21845); use BLOB or TEXT instead

mysql>  CREATE TABLE tr (id INT, name VARCHAR(65536), purchased DATE ) DEFAULT CHARSET=latin1;
ERROR 1074 (42000): Column length too big for column 'name' (max = 65535); use BLOB or TEXT instead

What is changed and how it works?

• Different character sets varchar type column maximum length limit is not the same.

• There are default settings for character sets and collations at four levels: server, database, table, and column.

1. First use the character set of Column to set the value;
2. If not set above, use the default character set value of the corresponding table;
3. If not set above, use the default character set value of the corresponding database;
4. If not set above, use the character_set_server setting.

Check List

Tests

• Unit test

---

This change is 

[ciscoxll]

@winkyao @zimulala @crazycs520 PTAL

[crazycs520]

Please fix tests.

[ciscoxll]

@winkyao @zimulala @crazycs520 PTAL

[ciscoxll]

@winkyao @zimulala @crazycs520 PTAL

[ciscoxll]

@winkyao @zimulala @crazycs520 PTAL

[ciscoxll]

@zimulala @crazycs520 PTAL

[codecov-io]

Codecov Report

Merging #8818 into master will decrease coverage by <.01%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #8818      +/-   ##
==========================================
- Coverage   67.27%   67.26%   -0.01%     
==========================================
  Files         371      371              
  Lines       75972    75997      +25     
==========================================
+ Hits        51108    51123      +15     
- Misses      20354    20362       +8     
- Partials     4510     4512       +2

Impacted Files	Coverage Δ
planner/core/preprocess.go	89.15% <100%> (+0.38%)	⬆️
ddl/ddl_api.go	75.06% <86.66%> (+0.21%)	⬆️
util/systimemon/systime_mon.go	80% <0%> (-20%)	⬇️
store/tikv/lock_resolver.go	41.7% <0%> (-0.95%)	⬇️
store/tikv/gcworker/gc_worker.go	41.14% <0%> (-0.43%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 53707ce...709777f. Read the comment docs.

[crazycs520]

LGTM

[tiancaiamao]

LGTM

[winkyao]

LGTM

[zimulala]

/run-all-tests

[ciscoxll]

/run-all-tests