How do I store orders?

Question

I have an app which has tasks in it and you can reorder them. Now I was woundering how to best store them. Should I have a colomn for the ordernumber and recalculate all of

Answer 1

This is not an easy problem. If you have a low number of sortable elements, I would just reset all of them to their new order.

Otherwise, it seems it would take just as much work or more to "test-and-set" to modify only the records that have changed.

You could delegate this work to the client-side. Have the client maintain old-sort-order and new-sort-order and determine which row[sort-order]'s should be updated - then passes those tuples to the PHP-mySQL interface.

You could enhance this method in the following way (doesn't require floats):

1. If all sortable elements in a list are initialized to a sort-order according to their position in the list, set the sort-order of every element to something like row[sort-order] = row[sort-order * K] where K is some number > average number of times you expect the list to be reordered. O(N), N=number of elements, but increases insertion capacity by at least N*K with at least K open slots between each exiting pair of elements.

2. Then if you want to insert an element between two others its as simple as changing its sort-order to be one that is > the lower element and < the upper. If there is no "room" between the elements you can simply reapply the "spread" algorithm (1) presented in the previous paragraph. The larger K is, the less often it will be applied.

The K algorithm would be selectively applied in the PHP script while the choosing of the new sort-order's would be done by the client (Javascript, perhaps).