util,executor: use MutableString as key for DecimalSet (#9913)

[XuHuaiyu]

cherry-pick #9913

What problem does this PR solve?

fix #9900 (comment)

What is changed and how it works?

Using MutableString as key for DecimalSet.
The root reason is the same as #9901.

Check List

Tests

• Integration test

Code changes

• Has exported function/method change

Side effects

• Possible performance regression
  I've tested it using tpch 10G dataset.

tidb [10.0.1.5]> desc lineitem;
+-----------------+---------------+------+------+---------+-------+
| Field           | Type          | Null | Key  | Default | Extra |
+-----------------+---------------+------+------+---------+-------+
...
| L_QUANTITY      | decimal(15,2) | NO   |      | NULL    |       |
...
+-----------------+---------------+------+------+---------+-------+

tidb [10.0.1.5]> select count(L_QUANTITY) from lineitem;
+-------------------+
| count(L_QUANTITY) |
+-------------------+
|          59986052 |
+-------------------+

tidb [10.0.1.5]> select count(distinct L_QUANTITY) from lineitem;
+----------------------------+
| count(distinct L_QUANTITY) |
+----------------------------+
|                         50 |
+----------------------------+

tidb [10.0.1.5]> select sum(distinct L_QUANTITY) from lineitem;

Before this commit(agg phase/ total phase)	After this commit(agg phase/ total phase)	performance regression(agg phase/ total phase)
7.21s/19s	12.86s/24.7s	78%/30%

Related changes

• Need to cherry-pick to the release branch

[XuHuaiyu]

/run-all-tests

[zz-jason]

LGTM

[SunRunAway]

LGTM

[sre-bot]

/run-all-tests