Data Structure for storing a sorting field to efficiently allow modifications

Question

I'm using Django and PostgreSQL, but I'm not absolutely tied to the Django ORM if there's a better way to do this with raw SQL or database specific operations.

I've got a model that needs sequential ordering. Lookup operations will generally retrieve the entire list in order. The most common operation on this data is to move a row to the bottom of a list, with a subset of the intervening items bubbling up to replace the previous item like this:

(operation on A, with subset B, C, E)

A -> B
B -> C
C -> E
D -> D
E -> A

Notice how D does not move.

In general, the subset of items will not be more than about 50 items, but the base list may grow to tens of thousands of entries.

The most obvious way of implementing this is with a simple integer order field. This seems suboptimal. It requires the compromise of making the position ordering column non-unique, where non-uniqueness is only required for the duration of a modification operation. To see this, imagine the minimal operation using A with subset B:

oldpos = B.pos
B.pos = A.pos
A.pos = oldpos

Even though you've stored the position, at the second line you've violated the uniqueness constraint. Additionally, this method makes atomicity problematic - your read operation has to happen before the write, during which time your records could change. Django's default transaction handling documentation doesn't address this, though I know it should be possible in the SQL using the "REPEATABLE READ" level of transaction locking.

I'm looking for alternate data structures that suit this use pattern more closely. I've looked at this question for ideas.

One proposal there is the Dewey decimal style solution, which makes insert operations occur numerically between existing values, so inserting A between B and C results in:

A=1   ->   B=2
B=2   ->   A=2.5
C=3   ->   C=3

This solves the column uniqueness problem, but introduces the issue that the column must be a float of a specified number of decimals. Either I over-estimate, and store way more data than I need to, or the system becomes limited by whatever arbitrary decimal length I impose. Furthermore, I don't expect use to be even over the database - some keys are going to be moved far more often than others, making this solution hit the limit sooner. I could solve this problem by periodically re-numbering the database, but it seems that a good data structure should avoid needing this.

Another structure I've considered is the linked list (and variants). This has the advantage of making modification straightforward, but I'm not certain of it's properties with respect to SQL - ordering such a list in the SQL query seems like it would be painful, and extracting a non-sequential subset of the list has terrible retrieval properties.

Beyond this, there are B-Trees, various Binary Trees, and so on. What do you recommend for this data structure? Is there a standard data structure for this solution in SQL? Is the initial idea of going with sequential integers really going to have scaling issues, or am I seeing problems where there are none?

Answer 1

It seems to me that your real problem is the need to lock a table for the duration of a transaction. I don't immediately see a good way to solve this problem in a single operation, hence the need for locking.

So the question is whether you can do this in a "Django way" as opposed to using straight SQL. Searching "django lock table" turned up some interesting links, including this snippet, there are many others that implement similar behavior.

A straight SQL linked-list style solution can be found in this stack overflow post, it appeared logical and succinct to me, but again it's two operations.

I'm very curious to hear how this turns out and what your final solution is, be sure to keep us updated!

Answer 2

You can solve the renumbering issue by doing the order column as an integer that is always an even number. When you are moving the data, you change the order field to the new sort value + 1 and then do a quick update to convert all the odd order fields to even:

update table set sort_order = bitand(sort_order, '0xFFFFFFFE')
where sort_order <> bitand(sort_order, '0xFFFFFFFE')

Thus you can keep the uniqueness of sort_order as a constraint

EDIT: Okay, looking at the question again, I've started a new answer.

Answer 3

Why not do a simple character field of some length like a max of 16 (or 255) initially.

Start initially with labeling things aaa through zzz (that should be 17576 entries). (You could also add in 0-9, and the uppercase letters and symbols for an optimization.)

As items are added, they can go to the end up to the maximum you allow for the additional 'end times' (zzza, zzzaa, zzzaaa, zzzaab, zzzaac, zzzaad, etc.)

This should be reasonable simple to program, and it's very similar to the Dewey Decimal system.

Yes, you will need to rebalance it occasionally, but that should be a simple operaion. The simplest approach is two passes, pass 1 would be to set the new ordering tag to '0' (or any character earlier than the first character) followed by the new tag of the appropriate length, and step 2 would be to remove the '0 from the front.

Obviuosly, you could do the same thing with floats, and rebalancing it regularly, this is just a variation on that. The one advantage is that most databases will allow you to set a ridiculously large maximum size for the character field, large enough to make it very, very, very unlikely that you would run out of digits to do the ordering, and also make it unlikely that you would ever have to modify the schema, while not wasting a lot of space.

Answer 4

Prefered solutions:

A linked list would be the usual way to achieve this. A query to return the items in order is trivial in Oracle, but Im not sure how you would do it in PostreSQL.

Another option would be to implement this using the ltree module for postgresql.

Less graceful (and write-heavy) solution: Start transaction. "select for update" within scope for row level locks. Move the target record to position 0, update the targets future succeeding records to +1 where their position is higher than the targets original position (or vice versa) and then update the target to the new position - a single additional write over that needed without a unique constraint. Commit :D

Simple (yet still write-heavy) solution if you can wait for Postgresql 8.5 (Alpha is available) :)

Wrap it in a transaction, select for update in scope, and use a deferred constraint (postgresql 8.5 has support for deferred unique constraints like Oracle).

Answer 5

A temp table and a transaction should maintain atomicity and the unique constraint on sort order. Restating the problem, you want to go from:

A  10   to  B  10
B  25       C  25
C  26       E  26
E  34       A  34

Where there can be any number of items in between each row. So, first you read in the records and create a list [['A',10],['B',25],['C',26],['E',34]]. Through some pythonic magic you shift the identifiers around and insert them into a temp table:

create temporary table reorder (
    id varchar(20), -- whatever
    sort_order number,
    primary key (id));

Now for the update:

update table XYZ
set sort_order = (select sort_order from reorder where xyz.id = reorder.id)
where id in (select id from reorder)

I'm only assuming pgsql can handle that query. If it can, it will be atomic.

Optionally, create table REORDER as a permanent table and the transaction will ensure that attempts to reorder the same record twice will be serialized.

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EDIT: There are some transaction issues. You might need to implement both of my ideas. If two processes both want to update item B (for example) there can be issues. So, assume all order values are even:

1. Begin Transaction
2. Increment all the orders being used by 1. This puts row level write locks on all the rows you are going to update.
3. Select the data you just updated, if any sort_order fields are even some other process has added a record that matches your criteria. You can either abort the transaction and restart or you can just drop the record and finish the operation using only the records that were updated in step 2. The "right" thing to do depends on what you need this code to accomplish.
4. Fill your temporary reorder table as above using the proper even sort_orders.
5. Update the main table as above.
6. Drop the temporary table.
7. Commit the transaction

Step 2 ensures that if two lists overlap, only the first one will have access to the row in question until the transaction completes:

update XYZ set sort_order = sort_order + 1
where -- whatever your select criteria are

select * from XYZ
where -- same select criteria
order by sort_order

Alternatively, you can add a control field to the table to get the same affect and then you don't need to play with the sort_order field. The benefit of using the sort_order field is indexing by a BIT field or a LOCK_BY_USERID field when the field is usually null tends to have poor performance since the index 99% of the time is meaningless. SQL engines don't like indexes that spend most of their time empty.