Re: unnesting multirange data types

On 6/13/21 11:49 AM, Justin Pryzby wrote:
> On Sun, Jun 13, 2021 at 11:25:05AM -0400, Jonathan S. Katz wrote:
>> On 6/13/21 10:57 AM, Zhihong Yu wrote:
>>> +/* Turn multirange into a set of ranges */
>>>
>>> set of ranges: sequence of ranges
>>
>> I believe "set of ranges" is accurate here, as the comparable return is
>> a "SETOF rangetype". Sequences are objects unto themselves.
>>
>
> I believe the point was that (in mathematics) a "set" is unordered, and a
> sequence is ordered. Also, a "setof" tuples in postgres can contain
> duplicates.

The comment in question is part of the header for the
"multirange_unnest" function in the code and AFAICT it is accurate: it
is returning a "set of" ranges as it's literally calling into the
set-returning function framework.

I would suggest leaving it as is.

> The docs say "The ranges are read out in storage order (ascending).", so I
> think this is just a confusion between what "set" means in math vs in postgres.

This is nearly identical to the language in the array unnest[1]
function, which is what I believed Alexander borrowed from:

"Expands an array into a set of rows. The array's elements are read out
in storage order."

If we tweaked the multirange "unnest" function, we could change it to:

+ <para>
+ Expands a multirange into a set of rows.
+ The ranges are read out in storage order (ascending).
+ </para>

to match what the array "unnest" function docs, or

+ <para>
+ Expands a multirange into a set of rows that each
+ contain an individual range.
+ The ranges are read out in storage order (ascending).
+ </para>

to be a bit more specific. However, I think this is also bordering on
overengineering the text, given there has been a lack of feedback on the
"unnest" array function description being confusing.

Thanks,

Jonathan

[1] https://www.postgresql.org/docs/current/functions-array.html