presto-main/src/main/java/io/prestosql/operator/scalar/StringFunctions.java/StringFunctions/stringPosition
class StringFunctions:    @Description("returns index of n-th occurrence of a substring (or 0 if not found)")
    @ScalarFunction("strpos")
    @LiteralParameters({"x", "y"})
    @SqlType(StandardTypes.BIGINT)
    public static long stringPosition(@SqlType("varchar(x)") Slice string, @SqlType("varchar(y)") Slice substring, @SqlType(StandardTypes.BIGINT) long instance)
    {
        if (instance < 0) {
            return stringPositionFromEnd(string, substring, abs(instance));
        }
        return stringPositionFromStart(string, substring, instance);
    }