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Hidden Indexes

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New in version 4.4.

Hidden indexes are not visible to the query planner and cannot be used to support a query.

By hiding an index from the planner, users can evaluate the potential impact of dropping an index without actually dropping the index. If the impact is negative, the user can unhide the index instead of having to recreate a dropped index.

Behavior

Apart from being hidden from the planner, hidden indexes behave like unhidden indexes; i.e.

  • If a hidden index is a unique index, the index still applies its unique constraint to the documents.
  • If a hidden index is a TTL index, the index still expires documents.
  • Hidden indexes are included in listIndexes and db.collection.getIndexes() results.
  • Hidden indexes are updated upon write operations to the collection and continue to consume disk space and memory. As such, they are included in various statistics operations, such as db.collection.stats() and $indexStats.
  • Hiding an unhidden index or unhiding a hidden index resets its $indexStats. Hiding an already hidden index or unhiding an already unhidden index does not reset the $indexStats.

Restrictions

Examples

Create a Hidden Index

To create a hidden index, use the db.collection.createIndex() method with the hidden option set to true.

Note

To use the hidden option with db.collection.createIndex(), you must have featureCompatibilityVersion set to 4.4 or greater. However, once hidden, the index remains hidden even with featureCompatibilityVersion set to 4.2 on MongoDB 4.4 binaries.

For example, the following operation creates a hidden ascending index on the borough field:

copy 
db.addresses.createIndex(
   { borough: 1 },
   { hidden: true }
);

To verify, run db.collection.getIndexes() on the addresses collection:

copy 
db.addresses.getIndexes()

The operation returns the following information:

[
   {
      "v" : 2,
      "key" : {
         "_id" : 1
      },
      "name" : "_id_"
   },
   {
      "v" : 2,
      "key" : {
         "borough" : 1
      },
      "name" : "borough_1",
      "hidden" : true
   }
]

The index option hidden is only returned if the value is true.

Hide an Existing Index

Note

To hide an existing index, you can use the collMod command or the mongo shell helper db.collection.hideIndex().

For example, create an index without hiding:

copy 
db.restaurants.createIndex( { borough: 1, ratings: 1 } );

To hide the index, you can specify either:

  • the index key specification document to the db.collection.hideIndex() method:

    copy 
    db.restaurants.hideIndex( { borough: 1, ratings: 1 } ); // Specify the index key specification document

  • the index name to the db.collection.hideIndex() method:

    copy 
    db.restaurants.hideIndex( "borough_1_ratings_1" );  // Specify the index name

To verify, run db.collection.getIndexes() on the restaurants collection:

copy 
db.restaurants.getIndexes()

The operation returns the following information:

[
   {
      "v" : 2,
      "key" : {
         "_id" : 1
      },
      "name" : "_id_"
   },
   {
      "v" : 2,
      "key" : {
         "borough" : 1,
         "ratings" : 1
      },
      "name" : "borough_1_ratings_1",
      "hidden" : true
   }
]

The index option hidden is only returned if the value is true.

Unhide an Existing Index

To unhide a hidden index, you can use the collMod command or the mongo shell helper db.collection.unhideIndex(). You can specify either:

  • the index key specification document to the db.collection.unhideIndex() method:

    copy 
    db.restaurants.unhideIndex( { borough: 1, city: 1 } );  // Specify the index key specification document

  • the index name to the db.collection.unhideIndex() method:

    copy 
    db.restaurants.unhideIndex( "borough_1_ratings_1" );    // Specify the index name

To verify, run db.collection.getIndexes() on the restaurants collection:

copy 
db.restaurants.getIndexes()

The operation returns the following information:

[
   {
      "v" : 2,
      "key" : {
         "_id" : 1
      },
      "name" : "_id_"
   },
   {
      "v" : 2,
      "key" : {
         "borough" : 1,
         "ratings" : 1
      },
      "name" : "borough_1_ratings_1"
   }
]

The index option hidden no longer appears as part of the borough_1_ratings_1 index since the field is only returned if the value is true.

Because indexes are fully maintained while hidden, the index is immediately available for use once unhidden.

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