code: func (c *Chunk) MemoryUsage() (sum int64) {
  	for _, col := range c.columns {
  		curColMemUsage := int64(unsafe.Sizeof(*col)) + int64(cap(col.nullBitmap)) + int64(cap(col.offsets)*4) + int64(cap(col.data)) + int64(cap(col.elemBuf))
  		sum += curColMemUsage
  	}
  	return
  }
docs: // MemoryUsage returns the total memory usage of a Chunk in B.
  // We ignore the size of column.length and column.nullCount
  // since they have little effect of the total memory usage.