presto-main/src/main/java/io/prestosql/operator/scalar/StringFunctions.java/StringFunctions/concat
class StringFunctions:    @Description("concatenates given character strings")
    @ScalarFunction
    @LiteralParameters({"x", "y", "u"})
    @Constraint(variable = "u", expression = "x + y")
    @SqlType("char(u)")
    public static Slice concat(@LiteralParameter("x") Long x, @SqlType("char(x)") Slice left, @SqlType("char(y)") Slice right)
    {
        int rightLength = right.length();
        if (rightLength == 0) {
            return left;
        }

        Slice paddedLeft = padSpaces(left, x.intValue());
        int leftLength = paddedLeft.length();

        Slice result = Slices.allocate(leftLength + rightLength);
        result.setBytes(0, paddedLeft);
        result.setBytes(leftLength, right);

        return result;
    }