Uploaded image for project: 'Core Server'
  1. Core Server
  2. SERVER-56757

$setWindowFields getting nondeterministic order of input would output nondeterministic result if ‘sortBy’ a non-existing field

    XMLWordPrintable

Details

    • Bug
    • Status: Closed
    • Major - P3
    • Resolution: Fixed
    • None
    • 5.0.0-rc1, 5.1.0-rc0
    • None
    • None
    • Fully Compatible
    • ALL
    • v5.0
    • Query Optimization 2021-05-17, Query Optimization 2021-05-31
    • 130

    Description

      If the preceding stages generate output in nondeterministic order and $setWindowFields has a default ‘partitionBy’ and ‘sortBy’ a non-existing field, $setWindowFields could generate inconsistent result every time the same pipeline is executed.

      Due to the same reason, disabling/enabling query optimization could also cause $setWindowFields producing inconsistent result. For example in BF-20745, consider a pipeline [*$bucket + $setWindowFields]* which will be desugared to the following pipeline: [ $group + $sort ({“_id”: 1})  + $sort + $_internalSetWindowFields ]. The two adjacent $sort stages will be optimized to a single $sort by removing the first $sort. However, if the second $sort sorted a non-existing field, $_internalSetWindowFields would still get a non-deterministic order of input because $group does not order its output.

       

      It seems not a legit $setWindowFields bug, but it’s failing in some certain agg-fuzzers, e.g. aggregation_wildcard_fuzzer and aggregation_optimization_fuzzer.

       

      Attachments

        Activity

          People

            samuel.mercier@mongodb.com Sam Mercier
            ruoxin.xu@mongodb.com Ruoxin Xu
            Votes:
            0 Vote for this issue
            Watchers:
            6 Start watching this issue

            Dates

              Created:
              Updated:
              Resolved: