code: func (b *builtinUncompressedLengthSig) evalInt(row chunk.Row) (int64, bool, error) {
  	sc := b.ctx.GetSessionVars().StmtCtx
  	payload, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return 0, true, err
  	}
  	if len(payload) == 0 {
  		return 0, false, nil
  	}
  	if len(payload) <= 4 {
  		// corrupted
  		sc.AppendWarning(errZlibZData)
  		return 0, false, nil
  	}
  	len := binary.LittleEndian.Uint32([]byte(payload)[0:4])
  	return int64(len), false, nil
  }
docs: // evalInt evals UNCOMPRESSED_LENGTH(str).
  // See https://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html#function_uncompressed-length