Builtin function ifnull() infers the wrong flen for param marker

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t;
create table t (c1 varchar(100), c2 varchar(128));
prepare pr1 from "insert into t values(ifnull(?,' '),ifnull(?,' '))";
set @a='1',@b=repeat('x', 80);
execute pr1 using @a,@b;

2. What did you expect to see? (Required)

mysql> execute pr1 using @a,@b;
Query OK, 1 row affected (0.01 sec)

3. What did you see instead (Required)

mysql> execute pr1 using @a,@b;
ERROR 1406 (22001): Data too long for column 'c2' at row 1

4. What is your TiDB version? (Required)

8db7c3b

[tangenta]

The root cause is that ifnull() infers the flen to 23(the max REAL length) even if the type is "ETString", which is apparently incorrect.

tidb/expression/builtin_control.go

Line 150 in 8db7c3b

resultFieldType.SetFlen(maxlen(lhs.GetFlen(), rhs.GetFlen()))