planner: cost trace for HashAgg in explain results is not correct

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> explain format='verbose' select /*+ hash_agg() */ count(*) from t;
+---------------------------+---------+---------+-----------+---------------+---------------------------------+
| id                        | estRows | estCost | task      | access object | operator info                   |
+---------------------------+---------+---------+-----------+---------------+---------------------------------+
| HashAgg_9                 | 1.00    | 0.00    | root      |               | funcs:count(Column#4)->Column#3 |
| └─TableReader_10          | 1.00    | 9.08    | root      |               | data:HashAgg_5                  |
|   └─HashAgg_5             | 1.00    | 42.68   | cop[tikv] |               | funcs:count(1)->Column#4        |
|     └─TableFullScan_8     | 3.00    | 119.00  | cop[tikv] | table:t       | keep order:false                |
+---------------------------+---------+---------+-----------+---------------+---------------------------------+
4 rows in set (0.00 sec)

The actual cost of this plan is correct, but the traced cost(estCost in the explain result) is not correct.

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

The cost of HashAgg_9 shouldn't be 0.

3. What did you see instead (Required)

It's 0.

4. What is your TiDB version? (Required)

mysql> select tidb_version();
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                                            |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.5.0-alpha-430-gc19e899b6
Edition: Community
Git Commit Hash: c19e899b6559ffbb0c5ab9c5c95fbf59289f5b07
Git Branch: master
UTC Build Time: 2022-02-28 13:37:46
GoVersion: go1.16.3
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+