types: fix a bug in decimal multiplication

[winkyao]

What have you changed? (mandatory)

Before this PR, we got the result from select 0.5999991229316*0.918755041726043 is not correct:

mysql> select 0.5999991229316*0.918755041726043;
+-----------------------------------+
| 0.5999991229316*0.918755041726043 |
+-----------------------------------+
|    0.551252217)0-++'-+4062276588 |
+-----------------------------------+
1 row in set (0.00 sec)

This is because we don't handle the overflow in DecimalMul, after this PR, we got:

mysql> select 0.5999991229316*0.918755041726043;
+-----------------------------------+
| 0.5999991229316*0.918755041726043 |
+-----------------------------------+
|    0.5512522192246113614062276588 |
+-----------------------------------+
1 row in set (0.08 sec)

What is the type of the changes? (mandatory)

• Bug fix (non-breaking change which fixes an issue)

How has this PR been tested? (mandatory)

UT

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

NO

Does this PR affect tidb-ansible update? (mandatory)

NO

Does this PR need to be added to the release notes? (mandatory)

yes

fix a bug in decimal multiplication.

Refer to a related PR or issue link (optional)

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

[winkyao]

@coocood @lysu PTAL

[coocood]

@winkyao
Can we replace all the add with add2?

[lysu]

/run-all-tests

[winkyao]

@coocood In MySQL, the ADD2 is only used in decimal_mul, I think we should keep the logic.

[lysu]

LGTM

[coocood]

LGTM

[lysu]

@AndreMouche maybe coprocess need fix this question too?

[AndreMouche]

no,we needn't @lysu