expression : fix bug in insert select union select.

[AilinKid]

What problem does this PR solve?

In TiDB:

create table bug (a varchar(100));

insert into bug select  ifnull(JSON_UNQUOTE(JSON_EXTRACT('[{"amount":2000,"feeAmount":0,"merchantNo":"20190430140319679394","shareBizCode":"20160311162_SECOND"}]', '$[0].merchantNo')),'') merchant_no union SELECT '20180531557' merchant_no;

Result：

ERROR 1406 (22001) : Data Too Long, field len 11, data len 20

Expected execute successfully!

What is changed and how it works?

This problem comes from jsonUnquoteFunction function.
We should fill the Flen in jsonUnquoteFunctionClass.getFunction().
Cause return type string is an element set (maybe autowrapped as an array) of whole json, so here we set the flen with mysql.MaxFieldVarCharLength.

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

Release note

• fix bug in insert select union select, resulting from data too long.

[codecov]

Codecov Report

Merging #13632 into master will decrease coverage by 0.4331%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##             master     #13632        +/-   ##
================================================
- Coverage   80.3499%   79.9168%   -0.4332%     
================================================
  Files           473        473                
  Lines        118117     115664      -2453     
================================================
- Hits          94907      92435      -2472     
- Misses        15857      15966       +109     
+ Partials       7353       7263        -90

[AilinKid]

/run-unit-test

[AilinKid]

/run-all-tests

[winkyao]

LGTM

[XuHuaiyu]

LGTM

[sre-bot]

cherry pick to release-3.0 failed

[sre-bot]

cherry pick to release-2.1 failed