avg overflow

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (a decimal(65,0), b decimal(65,30));
insert into t values (99999999999999999999999999999999999999999999999999999999999999999,99999999999999999999999999999999999.999999999999999999999999999999);
insert into t select * from t;
insert into t select * from t;
insert into t select * from t;


mysql> select * from t;
+-------------------------------------------------------------------+--------------------------------------------------------------------+
| a                                                                 | b                                                                  |
+-------------------------------------------------------------------+--------------------------------------------------------------------+
| 99999999999999999999999999999999999999999999999999999999999999999 | 99999999999999999999999999999999999.999999999999999999999999999999 |
| 99999999999999999999999999999999999999999999999999999999999999999 | 99999999999999999999999999999999999.999999999999999999999999999999 |
| 99999999999999999999999999999999999999999999999999999999999999999 | 99999999999999999999999999999999999.999999999999999999999999999999 |
| 99999999999999999999999999999999999999999999999999999999999999999 | 99999999999999999999999999999999999.999999999999999999999999999999 |
| 99999999999999999999999999999999999999999999999999999999999999999 | 99999999999999999999999999999999999.999999999999999999999999999999 |
| 99999999999999999999999999999999999999999999999999999999999999999 | 99999999999999999999999999999999999.999999999999999999999999999999 |
| 99999999999999999999999999999999999999999999999999999999999999999 | 99999999999999999999999999999999999.999999999999999999999999999999 |
| 99999999999999999999999999999999999999999999999999999999999999999 | 99999999999999999999999999999999999.999999999999999999999999999999 |
+-------------------------------------------------------------------+--------------------------------------------------------------------+

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

99999999999999999999999999999999999999999999999999999999999999999

3. What did you see instead (Required)

mysql> select avg(a) from t group by b;
ERROR 1105 (HY000): other error for mpp stream: DB::Exception: Decimal math overflow

4. What is your TiFlash version? (Required)

master

root cause

in the projection after rewritten avg = sum/count, the sum overflows decaim(65,0), but the div's return type is decimal(65,4), whose length of integer is shorter than 65. It throws exception overflow after div.

Start by running the SQL reproduction for AVG over the decimal(65,0) and decimal(65,30) columns. Trace the projection where AVG is rewritten as SUM/COUNT, focusing on the SUM decimal precision and the division return type described in the issue. Done means the query returns the expected average without a Decimal math overflow.