.shift() usage in nextDocument greatly slows down with large batchSize

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    • Type: Improvement
    • Resolution: Unresolved
    • Priority: Major - P3
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      https://github.com/mongodb/node-mongodb-native/blob/a7a3b99645ec76e4d653537b79263f00b3587e87/src/cursor/abstract_cursor.ts#L630 contains:

      const doc = cursor[kDocuments].shift();

      In V8, Array.prototype.shift() potentially copies the entirety of the array forward by one element, leading to bad performance for large batch sizes (which we tried using as part of investigating COMPASS-5455 – see attached CPU profile which points to unreasonably much time being spent in nextDocument). Specifically, fetching documents happens in O(nDocuments · batchSize) rather than the expectable O(nDocuments).

      If this is adjusted to perform plain array accesses instead of .shift() calls, performance is as expected.

        1. Screenshot from 2022-02-08 17-56-30.png
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          Anna Henningsen

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            Reporter:
            Anna Henningsen
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              Created:
              Updated: