presto-main/src/main/java/io/prestosql/operator/scalar/UrlFunctions.java/UrlFunctions/urlExtractParameter
class UrlFunctions:    @SqlNullable
    @Description("extract query parameter from url")
    @ScalarFunction
    @LiteralParameters({"x", "y"})
    @SqlType("varchar(x)")
    public static Slice urlExtractParameter(@SqlType("varchar(x)") Slice url, @SqlType("varchar(y)") Slice parameterName)
    {
        URI uri = parseUrl(url);
        if ((uri == null) || (uri.getRawQuery() == null)) {
            return null;
        }

        String parameter = parameterName.toStringUtf8();
        Iterable<String> queryArgs = QUERY_SPLITTER.split(uri.getRawQuery());

        for (String queryArg : queryArgs) {
            Iterator<String> arg = ARG_SPLITTER.split(queryArg).iterator();
            if (arg.next().equals(parameter)) {
                if (arg.hasNext()) {
                    return decodeUrl(arg.next());
                }
                // first matched key is empty
                return Slices.EMPTY_SLICE;
            }
        }

        // no key matched
        return null;
    }