planner: do not remove the first row func if it is a constant | tidb-test=pr/2277 (#50020)

[ti-chi-bot]

This is an automated cherry-pick of #50020

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

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