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  1. Core Server
  2. SERVER-58181

changestream fullDocument lookup projections introduce overheads in degenerate cases

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    • Icon: Improvement Improvement
    • Resolution: Unresolved
    • Icon: Major - P3 Major - P3
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    • Query Execution

    Description

      We are interested in projecting a subset of fields from the post-image document in a changestream.

      Testing showed (also see CLOUDP-91896) that performance only improves when the projection is smaller than some fraction of the total document size (as a function of network delay etc).

      When most fields in the document are projected, there could be a 30% slowdown.

      Is it possible to optimize changestream projection so that overheads for degenerate cases is minimized? Otherwise it is hard to determine whether or not Search can use this optimization for a certain collection.

      Note that in this case, the projection only applies to fields nested under `fullDocument.*`, so it is possible to push the projection down to the query system (by a query optimization or by exposing new APIs)

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            backlog-query-execution Backlog - Query Execution
            oren.ovadia@mongodb.com Oren Ovadia
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