Re: Issues with generate_series using integer boundaries

On 1 February 2011 21:32, Alban Hertroys
<dalroi@solfertje.student.utwente.nl> wrote:
> On 1 Feb 2011, at 21:26, Thom Brown wrote:
>
>> On 1 February 2011 01:05, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> Thom Brown <thom@linux.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
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Registered Linux user: #516935