Best representation of an ordered list in a database?

Question

I know that this sort of goes against the principles of a relational database but let me describe the situation.

I have a page where the user will place a number of

Answer 1

Solution: make index a string (because strings, in essence, have infinite "arbitrary precision"). Or if you use an int, increment index by 100 instead of 1.

The performance problem is this: there is no "in between" values between two sorted items.

item      index
-----------------
gizmo     1
              <<------ Oh no! no room between 1 and 2.
                       This requires incrementing _every_ item after it
gadget    2
gear      3
toolkit   4
box       5

Instead, do like this (better solution below):

item      index
-----------------
gizmo     100
              <<------ Sweet :). I can re-order 99 (!) items here
                       without having to change anything else
gadget    200
gear      300
toolkit   400
box       500

Even better: here is how Jira solves this problem. Their "rank" (what you call index) is a string value that allows a ton of breathing room in between ranked items.

Here is a real example of a jira database I work with

   id    | jira_rank
---------+------------
 AP-2405 | 0|hzztxk:
 ES-213  | 0|hzztxs:
 AP-2660 | 0|hzztzc:
 AP-2688 | 0|hzztzk:
 AP-2643 | 0|hzztzs:
 AP-2208 | 0|hzztzw:
 AP-2700 | 0|hzztzy:
 AP-2702 | 0|hzztzz:
 AP-2411 | 0|hzztzz:i
 AP-2440 | 0|hzztzz:r

Notice this example hzztzz:i. The advantage of a string rank is that you run out of room between two items, you still don't have to re-rank anything else. You just start appending more characters to the string to narrow down focus.

EDIT: as mentioned in the comments, you can't insert anything between 0|hzztzz: and 0|hzztzz:a. I guess that's why I see jira's database automatically append :i at the end regularly instead of :a to avoid that scenario. If you really want to prevent problems, then I think you can change your algorithm so that (for example) every time you would insert :a at the end, you instead insert :ai. This way you logically guarantee that no ranking will end with the letter a -- which should mean that you will always have "room" to insert more items without having to re-order anything.