coprocessor: avoid fmsketch calculation for single-column index

[xuyifangreeneyes]

What is changed and how it works?

Issue Number: ref #14231

What's Changed:

Implement pingcap/tidb#41931 in the tikv side. For the sinlge-column group, its fm_sketch is the same as that of the corresponding column. Hence, we don't maintain its fm_sketch in collect_column_group. We just copy the corresponding column's fm_sketch after iterating all rows. Also, we can directly copy total_size and null_count.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Manual test (add detailed scripts or steps below)

Side effects

Release note

None

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[REVIEW NOTIFICATION]

This pull request has been approved by:

• Xuanwo
• skyzh

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[xuyifangreeneyes]

/build

[xuyifangreeneyes]

/run-build

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[xuyifangreeneyes]

/cc @skyzh @AndreMouche @Xuanwo

[xuyifangreeneyes]

@skyzh Could you help trigger merge? Thx~

[skyzh]

/merge

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

Commit hash: 7aca843

[ti-chi-bot]

@xuyifangreeneyes: Your PR was out of date, I have automatically updated it for you.

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