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63 lines
3.1 KiB
Markdown
63 lines
3.1 KiB
Markdown
# Survey Of User-Defined Ordering Of Records
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### Approaches
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#### Positional Indexes
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Assign an integer position to everything in the list. To insert something in
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the middle of the list, you assign it that position and then increment the
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position each item in the list after it by one. Can require a lot more writes
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than other approaches.
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#### Positional Averaging
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You can insert an item between two others with a single operation by taking the
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average of the positional values (indexes) of the two surrounding items and
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making that the position of the item being inserted.
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- Decimal / Floating-Point - runs out of precision fairly quickly
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- Fractional / True Fractions - much more precision, trickier to implement
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- Large Int Boundaries - add first item at `0`, add second item at average of
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`0` and `MAX_INT`. Need to insert at the front of the list, average lowest
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index item with `MIN_INT`. [source](https://github.com/brendon/ranked-model/blob/master/lib/ranked-model.rb#L8-L11)
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#### LexoRank
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Similar to the (numeric) positional averaging approaches, but uses strings
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instead to get much more precision. Lexicographical ordering is used. To insert
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between two items, add a character to the end of the string that will position
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it between the two items.
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#### Stored Array
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Store an ordered array of element IDs either as a `jsonb` column or stringified
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JSON in a `text` column on the _collection_ table. If you have `list_items`
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that are ordered, then the `lists` table will have an `ordering` column of
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`list_items` IDs. You lose referential integrity with this approach.
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Here is an [example
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implementation](https://gist.github.com/jbranchaud/a00e0d6d17d562bec3007e3a4bcace94).
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#### Linked List
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A singly or doubly linked list where each item points to the next item in the list (and in the case of the doubly, points back to its previous item).
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[source](https://news.ycombinator.com/item?id=25802129)
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### References
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- [User-defined Order in SQL](https://begriffs.com/posts/2018-03-20-user-defined-order.html)
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- [Building Sortable Relations with PostgreSQL](https://brunoscheufler.com/blog/2022-09-26-building-sortable-relations-with-postgresql)
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- [User Defined Ordering Made Easy](https://steve.dignam.xyz/2020/03/31/practical-ordering/)
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- [Realtime Editing of Ordered Sequences](https://www.figma.com/blog/realtime-editing-of-ordered-sequences/#fractional-indexing/) (fractional indexing)
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- [Numeric and String Indexing](https://stackoverflow.com/questions/9536262/best-representation-of-an-ordered-list-in-a-database/49956113#49956113)
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- [Keeping an ordered collection in PostgreSQL](https://medium.com/the-missing-bit/keeping-an-ordered-collection-in-postgresql-9da0348c4bbe)
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- [Jira’s ranking system explained](https://tmcalm.nl/blog/lexorank-jira-ranking-system-explained/)
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- [JIRA LexoRank Explained](https://www.youtube.com/watch?v=OjQv9xMoFbg)
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- [User-defined Order in SQL | Hacker News](https://news.ycombinator.com/item?id=16635440)
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### Examples / Implementations
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- [Ranked Model - big integer positional averaging](https://github.com/brendon/ranked-model/blob/master/lib/ranked-model/ranker.rb#L174-L181)
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- [LexoRank Ruby Gem](https://github.com/richardboehme/lexorank)
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