统计信息使用场景
加载统计信息
从mysql.stats_*表中加载信息。
每个TiDB server有个goroutine 周期性的更新stat信息 Handle can update stats info periodically.
在TiDB启动时候，会启动一个goroutine, loadStatsWorker
Update, 更新statsCache
加载载table的Histogram和CMSketch tableStatsFromStorage
Selectivity
StatsNode
// StatsNode is used for calculating selectivity.
type StatsNode struct {
Tp int
ID int64
// mask is a bit pattern whose ith bit will indicate whether the ith expression is covered by this index/column.
mask int64
// Ranges contains all the Ranges we got.
Ranges []*ranger.Range
// Selectivity indicates the Selectivity of this column/index.
Selectivity float64
// numCols is the number of columns contained in the index or column(which is always 1).
numCols int
// partCover indicates whether the bit in the mask is for a full cover or partial cover. It is only true
// when the condition is a DNF expression on index, and the expression is not totally extracted as access condition.
partCover bool
}
// Selectivity is a function calculate the selectivity of the expressions.
// The definition of selectivity is (row count after filter / row count before filter).
// And exprs must be CNF now, in other words, `exprs[0] and exprs[1] and ... and exprs[len - 1]` should be held when you call this.
// Currently the time complexity is o(n^2).
Selectivity:
- 计算表达式的ranges: ExtractColumnsFromExpressions
questions:
- correlated column 是什么意思？
- maskCovered作用是什么
- statsNode的作用是什么
