code: func (c *getParamFunctionClass) getFunction(ctx sessionctx.Context, args []Expression) (builtinFunc, error) {
  	if err := c.verifyArgs(args); err != nil {
  		return nil, err
  	}
  	bf := newBaseBuiltinFuncWithTp(ctx, args, types.ETString, types.ETInt)
  	bf.tp.Flen = mysql.MaxFieldVarCharLength
  	sig := &builtinGetParamStringSig{bf}
  	return sig, nil
  }
docs: // getFunction gets function
  // TODO: more typed functions will be added when typed parameters are supported.