Re: explain analyze rows=%.0f

On 17.02.2025 17:19, Robert Haas wrote:
> On Mon, Feb 17, 2025 at 3:08 AM Ilia Evdokimov
> <ilya(dot)evdokimov(at)tantorlabs(dot)com> wrote:
>> You're right—floor shouldn't be used since it behaves differently across
>> platforms, as Tom also pointed out earlier. I like the idea of using %,
>> but since the compiler doesn't allow this operation with double, we need
>> to cast it to int64. I checked how nloops and ntuples are modified -
>> they are only incremented by 1. So that the casting might be safe. If I
>> missed anything or if there's a reason why this casting wouldn't be
>> safe, please let me know.
> What I've been advocating for is just:
>
> if (nloops > 1)
>
> Instead of:
>
> if (nloops > 1 && rows_is_fractonal)
>
> I don't think it's really safe to just cast a double back to int64. In
> practice, the number of tuples should never be large enough to
> overflow int64, but if it did, this result would be nonsense. Also, if
> the double ever lost precision, the result would be nonsense. If we
> want to have an exact count of tuples, we ought to change ntuples and
> ntuples2 to be uint64. But I don't think we should do that in this
> patch, because that adds a whole bunch of new problems to worry about
> and might cause us to get nothing committed. Instead, I think we
> should just always show two decimal digits if there's more than one
> loop.
>
> That's simpler than what the patch currently does and avoids this
> problem. Perhaps it's objectionable for some other reason, but if so,
> can somebody please spell out what that reason is so we can talk about
> it?
>

I have no objections. I agree if we are going to transition from double
to int64, it should be done properly, but definitely not in current thread.

As for documentation changes, I'll keep t1.unique > t2.unique. If we use
equality instead, we need to explicitly show rows=1.00, which would
likely raise too many questions from users what it means.

I attached patches v11 with changes.

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.