executor: using ToHashKey to check equality of decimal when count(distinct)

[XuHuaiyu]

What problem does this PR solve?

fix #9900

What is changed and how it works?

Using MyDecimal.ToHashKey to check the equality of decimal when in when count(distinct).

Before this commit, we wrap a cast as decimal(52, 31) for both children of union.
After the cast, the value from the int column becomes {9 3 3 false [1 0 0 0 0 0 0 0 0]}
the value from the decimal column becomes {3 3 3 false [1 0 0 0 0 0 0 0 0]}.
Which is thought as in-equal when compared directly.

Check List

Tests

• Integration test

Code changes

• Has exported function/method change

Side effects

• Possible performance regression

Before this commit, we use the value as a hash-key without evaluating.
This commit may cause some performance regression since we evaluating invoke MyDecimal.ToHashKey for every decimal.

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;

Before this commit(agg phase/ total phase)	After this commit(agg phase/ total phase)	performance regression(agg phase/ total phase)
12.9s/24.5s	16.5s/28.1s	14.69%/25.9%

Related changes

• Need to cherry-pick to the release branch

[codecov]

Codecov Report

Merging #9901 into master will increase coverage by 0.0138%.
The diff coverage is 50%.

@@               Coverage Diff                @@
##             master      #9901        +/-   ##
================================================
+ Coverage   77.2034%   77.2172%   +0.0138%     
================================================
  Files           405        405                
  Lines         81635      81667        +32     
================================================
+ Hits          63025      63061        +36     
+ Misses        13939      13929        -10     
- Partials       4671       4677         +6

[XuHuaiyu]

/run-all-tests tidb-test=pr/581

[XuHuaiyu]

/run-all-tests tidb-test=pr/581

[XuHuaiyu]

/run-all-tests tidb-test=pr/581

[winoros]

lgtm

What's the reason that we don't call EncodeKey here? For performance?

[XuHuaiyu]

They are almost the same, calling ToHashKey directly saves the cost of the function invocation. @winoros

[eurekaka]

LGTM