From: | Thom Brown <thom(at)linux(dot)com> |
---|---|
To: | Alban Hertroys <dalroi(at)solfertje(dot)student(dot)utwente(dot)nl> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PGSQL Mailing List <pgsql-general(at)postgresql(dot)org> |
Subject: | Re: Issues with generate_series using integer boundaries |
Date: | 2011-02-01 23:08:55 |
Message-ID: | AANLkTim7sgHDDcdFPnnyYBJGVLwdAXPuPpvDqyK4j63Z@mail.gmail.com |
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On 1 February 2011 21:32, Alban Hertroys
<dalroi(at)solfertje(dot)student(dot)utwente(dot)nl> wrote:
> On 1 Feb 2011, at 21:26, Thom Brown wrote:
>
>> On 1 February 2011 01:05, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Thom Brown <thom(at)linux(dot)com> writes:
>>>> I've noticed that if I try to use generate_series to include the upper
>>>> boundary of int4, it never returns:
>>>
>>> I'll bet it's testing "currval > bound" without considering the
>>> possibility that incrementing currval caused an overflow wraparound.
>>> We fixed a similar problem years ago in plpgsql FOR-loops...
>>
>> Yes, you're right. Internally, the current value is checked against
>> the finish. If it hasn't yet passed it, the current value is
>> increased by the step. When it reaches the upper bound, since it
>> hasn't yet exceeded the finish, it proceeds to increment it again,
>> resulting in the iterator wrapping past the upper bound to become the
>> lower bound. This then keeps it looping from the lower bound upward,
>> so the current value stays well below the end.
>
>
> That could actually be used as a feature to create a repeating series. A bit more control would be useful though :P
I don't quite understand why the code works. As I see it, it always
returns a set with values 1 higher than the corresponding result. So
requesting 1 to 5 actually returns 2 to 6 internally, but somehow it
correctly shows 1 to 5 in the query output. If there were no such
discrepancy, the upper-bound/lower-bound problem wouldn't exist, so
not sure how those output values result in the correct query result
values.
--
Thom Brown
Twitter: @darkixion
IRC (freenode): dark_ixion
Registered Linux user: #516935
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