"Data too long" error when dumping stats from a table with new collation

[time-and-fate]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

start a tidb with new collation enabled.

use test;
create table t(a varchar(10) collate utf8mb4_unicode_ci);
insert into t values('abcdefghij');
insert into t values('abcdefghnj');
analyze table t with 0 topn;

try to dump the stats:

http://127.0.0.1:10080/stats/dump/test/t

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

No error.

3. What did you see instead (Required)

"[types:1406]Data Too Long, field len 10, data len 20"

4. What is your TiDB version? (Required)

current master (af2da8d)

[time-and-fate]

/sig planner
/component statistics

[time-and-fate]

/assign time-and-fate

[ti-srebot]

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