JSON_TYPE result is turncated, charset max length should be considered

Hi, I'm gsoc-2022 contributor,I'm working on the MCOL-785.
Now I have a problem: My JSON_TYPE implementation return only the first 4 characters:

MariaDB root@(none):test> create table jt_test(i longtext) engine=columnstore;

Query OK, 0 rows affected

Time: 0.288s

MariaDB root@(none):test> insert into jt_test values('{}');

Query OK, 1 row affected

Time: 0.179s

MariaDB root@(none):test> insert into jt_test values("[]");

Query OK, 1 row affected

Time: 0.134s

MariaDB root@(none):test> insert into jt_test values(42);

Query OK, 1 row affected

Time: 0.116s

MariaDB root@(none):test> select json_type(i) from jt_test;

+--------------+

| json_type(i) |

+--------------+

| OBJE         |

| ARRA         |

| INTE         |

+--------------+

The charset used by json_type() Field is utf8_mb3. When the copy into the Field buffer takes place there is a copy length calculation that divides the length 12 bytes(strange but this is what it is) by 3(max byte per char length for utf8_mb3).So finally the result return 4 characters.

I found here that the max length and collation are set:

bool Item_func_json_type::fix_length_and_dec()

{

  collation.set(&my_charset_utf8mb3_general_ci);

  max_length= 12;

  set_maybe_null();

  return FALSE;

}

So I think the max_length should be set 12*collation.collation->mbmaxlen to make json_type in columnstore behave correctly.

• My JSON_TYPE implementation: func_json_type.cpp
• Pull Request: has been submitted and passed CI)
• The Callstack that might be useful (the #0、#1、#2、#3 from_length=6, nchars=4).