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  1. Documentation
  2. DOCS-9547

Docs for SERVER-19153: Conditionally push $match before $project

    • Type: Icon: Task Task
    • Resolution: Done
    • Priority: Icon: Major - P3 Major - P3
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      Engineering Ticket Description:

      I was looking at the optimizing aggregation pipelines docs page and didn't see this one listed.

      This is only possible if the subsequent $match doesn't use any computed fields, but there could be a large win with regards to index usage if you could move the $match prior to a $project.

      For example:

      { $project: { a: 1, b: 1, newField: { $add: ["$a", "$b"] } } },
      { $match: { a : { $gt: 10 } }
      

      is the same as

      { $match: { a : { $gt: 10 } },
      { $project: { a: 1, b: 1, newField: { $add: ["$a", "$b"] } } }
      

      because the $match is only using fields that exist in the original document.

            Assignee:
            nick.larew@mongodb.com Nicholas Larew
            Reporter:
            emily.hall Emily Hall
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              Created:
              Updated:
              Resolved:
              6 years, 39 weeks, 5 days ago