1. Improvement SERVER-55417
    Conditionally reorder $sort and $lookup depending on whether the query planner allows for an index-provided sort
  2. Improvement SERVER-48121
    Optimize a $group after a $unionWith
  3. Improvement SERVER-48120
    Optimize a $sort after a $unionWith
  4. Improvement SERVER-47466
    Optimize $push in $group to limit array size if followed by $slice of same field
  5. New Feature SERVER-42579
    combined $match and $project stages
  6. Bug SERVER-39938
    aggregation $match before $lookup optimization doesn't happen when $expr: $eq is used
  7. Improvement SERVER-37304
    Extend $sort+$group+$first pipeline optimization to $last
  8. Improvement SERVER-34741
    Move $match in front of $group if condition is on group key
  9. Improvement SERVER-33966
    redundant $sort in aggregation prevents best $limit $sort consolidation
  10. Improvement SERVER-33920
    Optimize transformation from BSON to Document in aggregation framework
  11. Improvement SERVER-31072
    reorder $limit before $lookup in pipeline
  12. Improvement SERVER-30474
    leading $facet with each facet beginning with $match should add $match before $facet
  13. Improvement SERVER-29010
    Coalesce adjacent exclusion $project stages and non-overlapping $addFields stages
  14. Improvement SERVER-28980
    aggregation can subsume $sort into $group when $first/$last are present
  15. Improvement SERVER-28942
    sort by shard key or prefix of shard key may not require merge before group
  16. New Feature SERVER-28592
    Move $sample earlier in the aggregation pipeline
  17. Bug SERVER-28454
    Make more of an effort to move $match stages through $redact stages
  18. Improvement SERVER-28312
    take slice or element of array before materializing it
  19. New Feature SERVER-27744
    merge sequential $project stages and $addFields stages when appropriate
  20. Improvement SERVER-27598
    when unwind includes "preserveNullAndEmptyArrays" limit stage that follows it can be also included before it
  21. Improvement SERVER-27389
    Use a covered index scan if $sample is the first stage and only the _id is needed
  22. Improvement SERVER-27283
    Sharded aggregations that need merging should only consider for merging the shards that have documents to contribute
  23. New Feature SERVER-26602
    $lookup and $graphLookup should only fetch fields needed
  24. Improvement SERVER-26442
    Push $sort before $project and $addFields
  25. Improvement SERVER-25957
    Optimize $filter + $arrayElemAt to avoid scanning entire array
  26. Improvement SERVER-25900
    Investigate converting TeeBuffer to return an entire batch at once, not one at a time
  27. Improvement SERVER-25897
    Add a priming stage to Pipeline execution to help determine if further optimization can happen.
  28. Bug SERVER-23991
    $graphLookup should optimize with $match
  29. Improvement SERVER-23033
    Implement getDependencies for $redacts that don't use $$DESCEND
  30. Improvement SERVER-22760
    Sharded aggregation pipelines which involve taking a simple union should merge on mongos
  31. Improvement SERVER-22631
    Improve efficiency of matching during an aggregation pipeline.
  32. Improvement SERVER-22404
    Extend MatchExpression to support matching Documents
  33. Improvement SERVER-22403
    Clean up dynamic_cast's in pipeline directory
  34. Improvement SERVER-22402
    Add dependency tracking to the $match stage
  35. Bug SERVER-22093
    Take advantage of the COUNT_SCAN optimization when a pipeline has no dependencies
  36. Improvement SERVER-21612
    Combine post $lookup $match on looked up field
  37. Improvement SERVER-21312
    $lookup should batch query requests
  38. Improvement SERVER-21284
    $lookup should cache query results
  39. Improvement SERVER-21200
    $match w/ $text should not use DocumentSource::setSource() to enforce pipeline position
  40. Improvement SERVER-21022
    Coalesce $group and $limit in aggregation pipeline
  41. Improvement SERVER-20506
    Conditionally order $match with $unwind
  42. Improvement SERVER-20378
    Clean up DocumentSourceUnwind::Unwinder, attempt to share code with $lookUp
  43. Improvement SERVER-20092
    Add ability for DocumentSourceCursor to coalesce with a $project stage if it is doing an equivalent projection
  44. Improvement SERVER-20066
    Query planner should consider index scans on empty query predicates
  45. Bug SERVER-19643
    Aggregation on unversioned connection should not use SHARDING_FILTER execution stage
  46. Improvement SERVER-19546
    Prevent $project's from being sent to shard and merge pipes.
  47. Improvement SERVER-19542
    Combine logic for all pipeline optimizations that involve swapping stages into one function
  48. Improvement SERVER-19400
    Move the checking for authorization from the pipeline level to the stage level.
  49. New Feature SERVER-19153
    Conditionally push $match before $project
  50. Bug SERVER-18940
    Optimise sharded aggregations that are targeted to a single shard
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