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Asked: 2026-06-17T08:05:36+00:00

Referencing the guide here, http://docs.mongodb.org/manual/core/indexes/ I cannot tell if Mongo indexes for fields are

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Referencing the guide here, http://docs.mongodb.org/manual/core/indexes/ I cannot tell if Mongo indexes for fields are stored persistently.

If ensureIndex() is called (and completes) within an application using MongoDB, what happens if:

  1. The application using MongoDB is restarted. Will a subsequent call to ensureIndex() cause a complete reindex?
  2. The MongoDB server is restarted. Would a later call of ensureIndex() from a client application rebuild?
  3. Is any of this affected by having multiple client sessions? I assume indexing is global across the entire collection per the documentation: "MongoDB defines indexes on a per-collection level."
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  1. Editorial Team

    The application using MongoDB is restarted. Will a subsequent call to ensureIndex() cause a complete reindex?

    No, it should (as in every other driver) register as a no op since the index already exists. Some drivers provide a cache mechanism to detect, without going to the server, if an index has been created (i.e. Python).

    The MongoDB server is restarted. Would a later call of ensureIndex() from a client application rebuild?

    Same as above

    Is any of this affected by having multiple client sessions? I assume indexing is global across the entire collection per the documentation: “MongoDB defines indexes on a per-collection level.”

    Yes indexes are stored in MongoDB on the collection itself (to be technical, as a namespace within the db.ns file). Since it is a single point of knowledge for ensureIndex and an index is a single process (much like the write lock really) multiple connections should not effect whether the index creation is registered twice.

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