Wrong behavior when ENUM column meet Aggregation

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table t(a enum('a', 'b'));
insert into t values('a'), ('b');
select cast(a as signed) from (select a from t group by a) t;

2. What did you expect to see?

Two row. 1 and 2.

3. What did you see instead?

Two row. Both are 0, with warning Data Truncated

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

current master 02b7634

[winoros]

max(a) and min(a) should also return the correct result without warning.

[pingyu]

Is it caused by (*baseFuncDesc).typeInfer4MaxMin ?

if a.RetTp.Tp == mysql.TypeEnum || a.RetTp.Tp == mysql.TypeSet {
	a.RetTp = &types.FieldType{Tp: mysql.TypeString, Flen: mysql.MaxFieldCharLength}
}

So we should add a typeInfer4FirstRow, which will infer to a same type as argument. Shouldn't we ?

PS. I found another issue when ENUM met aggregation: #13961

[winoros]

I think the one you found shares the same cause of this issue. You can have a try to do a cast as signed on the result after MAX/MIN.
If these two are the same cause, no need to add a new typeInfer4FirstRow. Change the original typeInfer4MaxMin is enough. (Also we need to change the execution logic).

[pingyu]

@winoros
Thanks for your advise. I had tried to fix it (#14035), would you please take a look ?