Add pagination with cursor

[mtrezza]

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

Query results pagination can currently only be achieved by using limit and skip. A MongoDB query however also returns a cursor that can be used to paginate through query results.

Feature / Enhancement Description

• Add the query result cursor to response of a query as meta data.
• Add a methods to interact with the cursor.

For example:

let q = new Parse.Query();
q.equalTo(...);
const response = await q.find();

const page1 = response.results;
const cursor = response.cursor;

q = Parse.Query.fromCursor(cursor);
q.skip(100);
q.limit(100);
const page2 = await q.find();

Or alternatively introduce an entirely new command set for cursor operations, for example:

c = Parse.QueryCursor(cursor);
c.moveToPosition(100);
const page2 = await c.return(100);

Returning meta data together with the response is related to a pending change of the response object for a sustainable modification of the response JSON structure, see #7440 (comment). Returning a cursor together with results is one use case that justifies the change of the query response from a ParseObject array to this more versatile form:

{
  results: [...],
  meta: {
    cursor: ...,
    explain: ...
  }
}

Example Use Case

See the current challenge of pagination in Parse Dashboard in parse-community/parse-dashboard#1551.

Alternatives / Workarounds

Paginate using limit and skip with the major downside of each pagination request issuing a new query instead of using the existing cached query results on the database side. The other challenge is that each new query is done over a potentially changing data set, so the limit / skip parameters become inaccurate.

[parse-github-assistant]

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[cbaker6]

Cursor for postgres: https://www.postgresql.org/docs/current/plpgsql-cursors.html