Re: UUID v7

On Sat, Nov 23, 2024 at 12:20 AM Andrey M. Borodin <x4mmm(at)yandex-team(dot)ru> wrote:
>
>
>
> > On 23 Nov 2024, at 10:58, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > I've attached an updated patch that squashed changes I made for v33.
> > We're still discussing increasing entropy on Windows and macOS, but
> > the patch seems to be in good shape.
>
> +1, thanks!
>
> PFA version with improved comment.

Thank you for updating the patch!

In the following code, we use "defined(__darwin__) || defined(_MSC_VER)":

+#if defined(__darwin__) || defined(_MSC_VER)
+#define SUBMS_MINIMAL_STEP_BITS 10
+#else
+#define SUBMS_MINIMAL_STEP_BITS 12
+#endif
#define SUBMS_BITS 12
-#define SUBMS_MINIMAL_STEP_NS ((NS_PER_MS / (1 << SUBMS_BITS)) + 1)
+#define SUBMS_MINIMAL_STEP_NS ((NS_PER_MS / (1 <<
SUBMS_MINIMAL_STEP_BITS)) + 1)

on the other hand, we use "defined(__darwin__) || defined(WIN32)" here:

+#if defined(__darwin__) || defined(WIN32)
+ /*
+ * On MacOS real time is truncted to microseconds. Thus, 2 least
+ * significant are dependent on other time-specific bits, thus
they do not
+ * contribute to uniqueness. To make these bit random we mix in two bits
+ * from CSPRNG.
+ *
+ * SUBMS_MINIMAL_STEP is chosen so that we still guarantee monotonicity
+ * despite altering these bits.
+ */
+ uuid->data[7] = uuid->data[7] ^ (uuid->data[8] >> 6);
+#endif

Is there a reason for using different macros?

In get_real_time_ns_ascending(), we use _MSC_VER so we use
clock_gettime() on MinGW.

>
> Sergey Prokhorenko just draw my attention to the new release of MariaDB [0]. They are doing very similar UUID v7 generation as we do [1].
>

Thank you for the references. It made me think that we can use the
function name uuid_v7() rather than uuidv7().

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com