planner: do not remove the first row func if it is a constant | tidb-test=pr/2277

[Rustin170506]

What problem does this PR solve?

Issue Number: close #38756

Problem Summary:

What changed and how does it work?

Do not remove the first row func if the group by expr is a constant.

For instance, if you try SELECT DISTINCT SQRT(1) FROM t

mysql> explain SELECT DISTINCT SQRT(1) FROM t;
+---------------------------+----------+-----------+---------------+-------------------------------------------------------+
| id                        | estRows  | task      | access object | operator info                                         |
+---------------------------+----------+-----------+---------------+-------------------------------------------------------+
| HashAgg_8                 | 1.00     | root      |               | group by:Column#5, funcs:firstrow(Column#5)->Column#3 |
| └─TableReader_9           | 1.00     | root      |               | data:HashAgg_4                                        |
|   └─HashAgg_4             | 1.00     | cop[tikv] |               | group by:1,                                           |
|     └─TableFullScan_7     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo                        |
+---------------------------+----------+-----------+---------------+-------------------------------------------------------+
4 rows in set (0.00 sec)

As you can see, we accidentally removed the first-row func because the group by expr is a constant 1 and the arg is also 1.

But if you try SELECT DISTINCT SQRT(2) FROM t, it works well.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix the TiDB issue with incorrect results when the function's argument matches the group by key

[codecov]

Codecov Report

Merging #50020 (c917924) into master (eaab02f) will decrease coverage by 14.6528%.
Report is 3 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #50020         +/-   ##
=================================================
- Coverage   70.0660%   55.4132%   -14.6528%     
=================================================
  Files          1443       1554        +111     
  Lines        419720     586645     +166925     
=================================================
+ Hits         294081     325079      +30998     
- Misses       105346     238848     +133502     
- Partials      20293      22718       +2425     

Flag	Coverage Δ
integration	36.6022% <0.0000%> (?)
unit	70.0199% <100.0000%> (-0.0461%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	54.0269% <ø> (ø)
parser	∅ <ø> (∅)
br	55.5035% <ø> (+5.8242%)	⬆️

[Rustin170506]

/test all

[Rustin170506]

🔢 Self-check

[Rustin170506]

/retest

[Rustin170506]

/retest

[AilinKid]

/test mysql-test

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, elsa0520, fixdb

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[Rustin170506]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.4: #50283.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #50284.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #50285.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #50286.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #50287.