Re: Resources on modeling ordered hierachies?

On 4/7/22 10:55, Peter J. Holzer wrote:
> On 2022-04-07 17:45:49 +1200, Tim Uckun wrote:
>> There a tons of articles about how to model hierarchies in SQL but I
>> haven't seen any about dealing with hierarchies where the order of
>> children is important.
>>
>> The canonical example is a simple outline
>>
>> 1.
>> 1.1
>> 1.1.1
>> 1.2
>> 2.
>> 2.1
>>
>> etc
>>
>> If I am doing an insert where parent is 1.1 it should name it 1.1.2
>> which to me means doing something like select max(id) + 1 where parent
>> = 1.1 or something like that which might turn out to be expensive.
> Shouldn't be that bad with an appropriate index. Bigger problem might be
> that two transactions could attempt this at the same time.
>
>
>> Similarly if I want to insert something between 1.1 and 1.2 I need to
>> do something like update id set id = id+1 where parent = 1 and id >1
>
> You can mostly get around that by using float8 or even numeric instead
> of int. Chances are that there is a free number between you numbers.
>
> hp
>
Is there any chance the 1/1.1/1.1.1 stuff can be cosmetic/generated?
Corollary: what determines the hierarchical position of the next record?

Each item could know its parent (nullable) and it next-sib (nullable).
If, as your example might suggest, you are always adding to the end,
then you're updating the last sib's null next-sib point and the incoming
record has next-sib null.  All the single numbers are parentless, 2 is
next sib of 1.  Trickier to interject a new record between to sibs but
not impractical.  Can move sibships up/down hierarchy by updating parent
and resetting next-sib of last in moved sibship and the next-sib of the
injection point to the head of the move sibs.