planner: reduce topn count to exclude non-skewed values | tidb-test=pr/2333

[terry1purcell]

What problem does this PR solve?

Issue Number: close #53111

Problem Summary:

What changed and how does it work?

PR includes 3 changes to reduce unnecessary TopN values:

1. Reduce the default TopN to 100 (from 500).
2. Support pruning of TopN values that are "not skewed" for smaller tables.
3. And initially avoid adding values with a count=1 (which will be pruned anyway).

Check List

Tests

• [x] 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.

None

[ti-chi-bot[bot]]

Hi @terry1purcell. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[hawkingrei]

/ok-to-test

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.5481%. Comparing base (8483dc5) to head (132878c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #53035 +/- ## ================================================ + Coverage 72.4842% 74.5481% +2.0639% ================================================ Files 1506 1506 Lines 431000 431041 +41 ================================================ + Hits 312407 321333 +8926 + Misses 99193 89782 -9411 - Partials 19400 19926 +526 ``` | [Flag](https://app.codecov.io/gh/pingcap/tidb/pull/53035/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/pingcap/tidb/pull/53035/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `49.3713% <78.5714%> (?)` | | | [unit](https://app.codecov.io/gh/pingcap/tidb/pull/53035/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `71.4249% <100.0000%> (-0.0296%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap#carryforward-flags-in-the-pull-request-comment) to find out more. | [Components](https://app.codecov.io/gh/pingcap/tidb/pull/53035/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [dumpling](https://app.codecov.io/gh/pingcap/tidb/pull/53035/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `53.9957% <ø> (ø)` | | | [parser](https://app.codecov.io/gh/pingcap/tidb/pull/53035/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `∅ <ø> (∅)` | | | [br](https://app.codecov.io/gh/pingcap/tidb/pull/53035/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `50.4537% <ø> (+8.7018%)` | :arrow_up: |

[ti-chi-bot[bot]]

@ZL91: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to [this](https://github.com/pingcap/tidb/pull/53035#pullrequestreview-2065073395): > Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ZL91 Once this PR has been reviewed and has the lgtm label, please assign elsa0520 for approval. For more information see the Code Review Process.

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Needs approval from an approver in each of these files: - **[OWNERS](https://github.com/pingcap/tidb/blob/master/OWNERS)** - **[pkg/planner/OWNERS](https://github.com/pingcap/tidb/blob/master/pkg/planner/OWNERS)** - **[pkg/statistics/OWNERS](https://github.com/pingcap/tidb/blob/master/pkg/statistics/OWNERS)** Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment