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  1. Core Server
  2. SERVER-15279

Disable hash-based index intersection (AND_HASH) by default

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    Details

    • Type: Task
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 2.6.5, 2.7.7
    • Component/s: Querying
    • Labels:
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    • Backport Completed:
    • Case:

      Description

      Performance testing has suggested that hash-based index intersection, implemented by the AND_HASH query execution stage, does not offer a significant performance advantage for test workloads. This is due in part to SERVER-12923, which describes how query optimization can suffer for plans that are not fully pipelined.

      Based on the results of this testing, we should disable hash-based intersection by default. It will be possible to re-enable AND_HASH at run time via:

      db.adminCommand({setParameter: 1, internalQueryPlannerEnableHashIntersection: 1});

      Sort-based intersection will be on by default and will not be affected by this change.

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