statistics: remove statistics.Column.Count (#43033)

[ti-chi-bot]

This is an automated cherry-pick of #43033

What problem does this PR solve?

Issue Number: ref #42160 close #44404

Problem Summary:

What is changed and how it works?

Remove statistics.Column.Count.

Before the PR, in order to maintain statistics.Column.Count, we need to read mysql.stats_top_n and mysql.stats_buckets for each column when init stats, which is time-consuming. On the other hand, the usage of statistics.Column.Count is limited. We modify (*DataSource).getColumnNDV to get rid of statistics.Column.Count.

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.

None

[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	3848af8	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	3848af8	link	true	/test check-dev
idc-jenkins-ci-tidb/check_dev_2	3848af8	link	true	/test check-dev2
idc-jenkins-ci-tidb/build	3848af8	link	true	/test build
idc-jenkins-ci-tidb/mysql-test	3848af8	link	true	/test mysql-test

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-05 06:44:55.972911117 +0000 UTC m=+244282.245883920: ☑️ agreed by hawkingrei.

[xuyifangreeneyes]

Too much conflict. I will manually do the cherry pick(#44406).