executor, util: fix UnionScan Next() skip reading data when passed chunk capacity is 0 (#36961)

[ti-srebot]

cherry-pick #36961 to release-6.1
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---

What problem does this PR solve?

Issue Number: close #36903

Problem Summary:

What is changed and how it works?

In the old UnionScan executor code, if an empty chk is passed to UnionScan.Next(chk) , the UnionScan executor will not read any data.

This bug could be trigger in select pkey from t_vwvgdc where 0 <> 0 union select pkey from t_vwvgdc;
The union first children return empty chunk, and the chunk is reused for the second children (union scan) to read data.

mysql> explain select pkey from t_vwvgdc where 0 <> 0 union select pkey from t_vwvgdc;
+------------------------------+---------+-----------+----------------+-------------------------------------------------------+
| id                           | estRows | task      | access object  | operator info                                         |
+------------------------------+---------+-----------+----------------+-------------------------------------------------------+
| HashAgg_13                   | 24.20   | root      |                | group by:Column#7, funcs:firstrow(Column#7)->Column#7 |
| └─Union_14                   | 29.00   | root      |                |                                                       |
|   ├─TableDual_16             | 0.00    | root      |                | rows:0                                                |
|   └─UnionScan_18             | 29.00   | root      |                |                                                       |
|     └─TableReader_20         | 29.00   | root      |                | data:TableFullScan_19                                 |
|       └─TableFullScan_19     | 29.00   | cop[tikv] | table:t_vwvgdc | keep order:false, stats:pseudo                        |
+------------------------------+---------+-----------+----------------+-------------------------------------------------------+
6 rows in set (0.00 sec)

Now I change req.GrowAndReset(us.maxChunkSize) to ensure capacity > 0 for the req.

Check List

Tests

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

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 a bug that UnionScan's Next() function skips reading data when the passed chunk's capacity is 0

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• qw4990
• xhebox

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[ti-srebot]

/run-all-tests

[ti-srebot]

@tiancaiamao you're already a collaborator in bot's repo.

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/6dbf4057cb7ce8740a500c5ed77c7b9f0c4e7c64

[wjhuang2016]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 6dbf405

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