code: func (c *column) appendMultiSameNullBitmap(notNull bool, num int) {
  	numNewBytes := ((c.length + num + 7) >> 3) - len(c.nullBitmap)
  	b := byte(0)
  	if notNull {
  		b = 0xff
  	}
  	for i := 0; i < numNewBytes; i++ {
  		c.nullBitmap = append(c.nullBitmap, b)
  	}
  	if !notNull {
  		c.nullCount += num
  		return
  	}
  	// 1. Set all the remaining bits in the last slot of old c.numBitMap to 1.
  	numRemainingBits := uint(c.length % 8)
  	bitMask := byte(^((1 << numRemainingBits) - 1))
  	c.nullBitmap[c.length/8] |= bitMask
  	// 2. Set all the redundant bits in the last slot of new c.numBitMap to 0.
  	numRedundantBits := uint(len(c.nullBitmap)*8 - c.length - num)
  	bitMask = byte(1<<(8-numRedundantBits)) - 1
  	c.nullBitmap[len(c.nullBitmap)-1] &= bitMask
  }
docs: // appendMultiSameNullBitmap appends multiple same bit value to `nullBitMap`.
  // notNull means not null.
  // num means the number of bits that should be appended.