statistics: refine index/column stats methods and remove statistics.Column.Count #44406
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This is a manual cherry-pick of #42910 and #43033
What problem does this PR solve?
Issue Number: ref #42160 close #44404
Problem Summary:
What is changed and how it works?
#42910
#43033
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.
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