Support _skip parameter for pagination

[almostchristian]

Paging with FHIR server currently uses a continuation token which is not meant to be directly manipulated by the client. This limits traditional paging GUIs other than an infinite scrolling list.

Ideally it should look something like below:

first page:  Patient
second page: Patient?_skip=10
third page:  Patient?_skip=20

I created a POC to enable this for the SQL Server provider. Please have a look.
https://github.com/almostchristian/fhir-server/tree/fhir/paging

The change involves converting the usages of TOP into OFFSET

  SELECT DISTINCT T1, Sid1, 1 AS IsMatch, 0 AS IsPartial 
  FROM cte1
  ORDER BY Sid1 ASC
  OFFSET (@p4) ROWS
  FETCH NEXT @p5 ROWS ONLY

A lot of change had to be done to support sorting, which is still tricky. Since sorting can happen in two phases, the skip parameter in the POC resets when the second phase starts. I denote the second phase with a hyphen in the skip parameter,

The search urls when sorting is involved can look like below. In here, repurposed ct as the skip parameter.

1st page: Patient?_sort=family&_count=2
2nd page: Patient?_sort=family&_count=2&ct=2
3rd page: Patient?_sort=family&_count=2&ct=-1
4th page: Patient?_sort=family&_count=2&ct=-3

Since I am currently working with a fork of FHIR server, I plan to go introduce this to our implementation. The sorting issue is a bummer, but I don't think this can be avoided without a big redesign. I'd like to hear feedback on this implementation. The E2E integration tests all pass, but I'm not sure if there is some edge cases not covered in the integration tests that could break this.

In terms of performance, the generated query plan is the same with the original, but the OFFSET version seems to be heavier compared to the TOP version outside of the first page, but I'd need to do proper testing on a large db.

AB#105830

[EXPEkesheth]

@almostchristian - Apologizes on delayed response. We would be interested to review.

[almostchristian]

@EXPEkesheth I created a draft PR here #3536

[EXPEkesheth]

Thanks @almostchristian - have scheduled it in our queue for review. We will respond back once we have reviewed or have additional questions.

[EXPEkesheth]

@almostchristian
Thanks for the time taken to submit the review. We reviewed the change and unfortunately, we won't be able to accept the change in the current form. The proposed change is not aligned with MSFT FHIR server approach with using snapshots for continuation token. The proposed change introduces a new behavior with implementing separate parameter from the continuation token. Please update the PR to align with MSFT FHIR server approach.