Re: UUID v7

Sorry for this long reply. I was looking on refactoring around pg_strong_random() and could not decide what to do. Finally, I decided to post at least something.

> On 22 Mar 2024, at 19:15, Peter Eisentraut <peter@eisentraut.org> wrote:
> 
> I have been studying the uuidv() function.
> 
> I find this code extremely hard to follow.
> 
> We don't need to copy all that documentation from the RFC 4122bis document.  People can read that themselves.  What I would like to see is easy to find information what from there we are implementing.  Like,
> 
> - UUID version 7
> - fixed-length dedicated counter
> - counter is 18 bits
> - 4 bits are initialized as zero

I've removed table taken from RFC.

> That's more or less all I would need to know what is going on.
> 
> That said, I don't understand why you say it's an 18 bit counter, when you overwrite 6 bits with variant and version.  Then it's just a 12 bit counter?  Which is the size of the rand_a field, so that kind of makes sense.  But 12 bits is the recommended minimum, and (in this patch) we don't use sub-millisecond timestamp precision, so we should probably use more than the minimum?


No, we use 4 bits in data[6], 8 bits in data[7], and 6 bits data[8]. It's 18 total. Essentially, we use both partial bytes and one whole byte between.
There was a bug - we used 1 extra byte of random numbers that was not necessary, I think that's what lead you to think that we use 12-bit counter.

> Also, you are initializing 4 bits (I think?) to zero to guard against counter rollovers (so it's really just an 8 bit counter?).  But nothing checks against such rollovers, so I don't understand the use of that.

No, there's only one guard rollover bit.
Here: uuid->data[6] = (uuid->data[6] & 0xf7);
Bits that are called "guard bits" do not guard anything, they just ensure counter capacity when it is initialized.
Rollover is carried into time tick here: 
	++sequence_counter;
        if (sequence_counter > 0x3ffff)
        {
            /* We only have 18-bit counter */
            sequence_counter = 0;
            previous_timestamp++;
        }

I think we might use 10 bits of microseconds and have 8 bits of a counter. Effect of a counter won't change much. But I'm not sure if this is allowed per RFC.
If time source is coarse-grained it still acts like a random initializer. And when it is precise - time is "natural" source of entropy.


> The code code be organized better.  In the not-increment_counter case, you could use two separate pg_strong_random calls: One to initialize rand_b, starting at &uuid->data[8], and one to initialize the counter. Then the former could be shared between the two branches, and the code to assign the sequence_counter to the uuid fields could also be shared.

Call to pg_strong_random() is very expensive in builds without ssl (and even with ssl too). If we could ammortize random numbers in small buffers - that would save a lot of time (see v8-0002-Buffer-random-numbers.patch upthread). Or, perhaps, we can ignore cost of two pg_string_random() calls.

> 
> I would also prefer if the normal case (not-increment_counter) were the first if branch.

Done.

> Some other notes on your patch:
> 
> - Your rebase duplicated the documentation of uuid_extract_timestamp and uuid_extract_version.
> 
> - PostgreSQL code uses uint64 etc. instead of uint64_t etc.
> 
> - It seems the added includes
> 
> #include "access/xlog.h"
> #include "utils/builtins.h"
> #include "utils/datetime.h"
> 
> are not needed.
> 
> - The static variables sequence_counter and previous_timestamp could be kept inside the uuidv7() function.

Fixed.

Thanks!


Best regards, Andrey Borodin.