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executor, util: fix UnionScan Next() skip reading data when passed chunk capacity is 0 (#36961)#37335

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executor, util: fix UnionScan Next() skip reading data when passed chunk capacity is 0 (#36961)#37335
ti-srebot wants to merge 1 commit intopingcap:release-6.2from
ti-srebot:release-6.2-81a93a6977e5

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@ti-srebot ti-srebot commented Aug 24, 2022 •
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cherry-pick #36961 to release-6.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/37335

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/37335:release-6.2-81a93a6977e5

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

@tiancaiamao @ti-srebot
cherry pick pingcap#36961 to release-6.2
cc6b408 
Signed-off-by: ti-srebot <ti-srebot@pingcap.com>
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ti-chi-bot commented Aug 24, 2022 •
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[REVIEW NOTIFICATION]

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  • xhebox

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ti-srebot commented Aug 24, 2022

/run-all-tests

@ti-chi-bot ti-chi-bot added do-not-merge/release-note-label-needed Indicates that a PR should not merge because it's missing one of the release note labels. release-note Denotes a PR that will be considered when it comes time to generate release notes. and removed do-not-merge/release-note-label-needed Indicates that a PR should not merge because it's missing one of the release note labels. labels Aug 24, 2022
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@ti-srebot ti-srebot added size/S Denotes a PR that changes 10-29 lines, ignoring generated files. type/6.2-cherry-pick labels Aug 24, 2022
@ti-srebot ti-srebot requested review from qw4990 and xhebox August 24, 2022 05:26
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ti-srebot commented Aug 24, 2022

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

@ti-chi-bot ti-chi-bot added the status/LGT1 Indicates that a PR has LGTM 1. label Aug 24, 2022
@VelocityLight VelocityLight added cherry-pick-approved Cherry pick PR approved by release team. and removed do-not-merge/cherry-pick-not-approved labels Aug 25, 2022
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tiancaiamao commented Aug 25, 2022

6.2 is not LTS version

@tiancaiamao tiancaiamao deleted the release-6.2-81a93a6977e5 branch August 25, 2022 05:30
@VelocityLight VelocityLight added do-not-merge/cherry-pick-not-approved and removed cherry-pick-approved Cherry pick PR approved by release team. labels Aug 25, 2022
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@xhebox xhebox xhebox approved these changes

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