code: func NewHistogram(id, ndv, nullCount int64, version uint64, tp *types.FieldType, bucketSize int, totColSize int64) *Histogram {
  	return &Histogram{
  		ID:                id,
  		NDV:               ndv,
  		NullCount:         nullCount,
  		LastUpdateVersion: version,
  		Tp:                tp,
  		Bounds:            chunk.NewChunkWithCapacity([]*types.FieldType{tp}, 2*bucketSize),
  		Buckets:           make([]Bucket, 0, bucketSize),
  		TotColSize:        totColSize,
  	}
  }
docs: // NewHistogram creates a new histogram.