improve index merge row count estimation

[time-and-fate]

Development Task

Currently, when index merge estimates total row count of all partial paths, it simply adds up row counts of all partial paths as implemented below.

tidb/planner/core/find_best_task.go

Lines 769 to 780 in 8359f67

	for _, partPath := range path.PartialIndexPaths {
	var scan PhysicalPlan
	var partialCost, rowCount float64
	if partPath.IsTablePath() {
	scan, partialCost, rowCount = ds.convertToPartialTableScan(prop, partPath)
	} else {
	scan, partialCost, rowCount = ds.convertToPartialIndexScan(prop, partPath)
	}
	scans = append(scans, scan)
	totalCost += partialCost
	totalRowCount += rowCount
	}

This will cause inaccurate row count estimation because there may be overlapping. We need to improve index merge row count estimation method.

[time-and-fate]

/assign @time-and-fate