code: func splitCompositeKeyOfPurgeIndexByHeight(compositeKey []byte) (txid string, uuid string, blockHeight uint64) {
  	var n int
  	blockHeight, n = util.DecodeOrderPreservingVarUint64(compositeKey[2:])
  	splits := bytes.Split(compositeKey[n+3:], []byte{compositeKeySep})
  	txid = string(splits[0])
  	uuid = string(splits[1])
  	return
  }
docs: // splitCompositeKeyOfPurgeIndexByHeight splits the compositeKey (<purgeIndexByHeightPrefix>~blockHeight~txid~uuid)
  // into txid, uuid and blockHeight.