Docs Menu

Docs HomeMongoDB Manual

Manage Indexes

On this page

  • View Existing Indexes
  • Remove Indexes
  • Modify an Index
  • Find Inconsistent Indexes Across Shards

This page shows how to manage existing indexes. For instructions on creating indexes, refer to the specific index type pages.

Tip

Hide an Index Before Dropping It

If you drop an index that is actively used in production, your application may incur a performance degradation. Before you drop an index, you can evaluate the potential impact of the drop by hiding the index.

Hidden indexes are not used to support queries. If you hide an index and observe substantial negative performance impact, consider keeping and unhiding the index so queries can resume using it.

If you drop an index that is actively used in production, your application may incur a performance degradation. To ensure queries can still use an index during modification, you can create a temporary, redundant index that contains the same fields as the modified index.

This example creates a new index and modifies that index to make it unique.

1

Run this command:

db.siteAnalytics.createIndex( { "url": 1 } )

The command returns the name of the index:

url_1
2

Run this command:

db.siteAnalytics.createIndex( { "url": 1, "dummyField": 1 } )

The command returns the name of the index:

url_1_dummyField_1

This temporary index lets you safely drop the original { "url": 1 } index without impacting performance.

3

Run this command:

db.siteAnalytics.dropIndex( { "url": 1 } )

The command returns:

{ nIndexesWas: 3, ok: 1 }
4

Run this command:

db.siteAnalytics.createIndex( { "url": 1 }, { "unique": true } )

The command returns the name of the index:

url_1

The url_1 index is recreated and you can drop the temporary index without impacting performance. Queries on the url field can use the new unique index.

5

Run this command:

db.siteAnalytics.dropIndex( { "url": 1, "dummyField": 1 } )

The command returns:

{ nIndexesWas: 3, ok: 1 }
6

To view the indexes on the siteAnalytics collection, run this command:

db.siteAnalytics.getIndexes()

The command returns these indexes, indicating that the url_1 index is now unique:

[
{ v: 2, key: { _id: 1 }, name: '_id_' },
{ v: 2, key: { url: 1 }, name: 'url_1', unique: true }
]

A sharded collection has an inconsistent index if the collection does not have the exact same indexes (including the index options) on each shard that contains chunks for the collection. Although inconsistent indexes should not occur during normal operations, inconsistent indexes can occur , such as:

  • When a user is creating an index with a unique key constraint and one shard contains a chunk with duplicate documents. In such cases, the create index operation may succeed on the shards without duplicates but not on the shard with duplicates.

  • When a user is creating an index across the shards in a rolling manner (i.e. manually building the index one by one across the shards) but either fails to build the index for an associated shard or incorrectly builds an index with different specification.

Starting in MongoDB 4.4 (and 4.2.6), the config server primary, by default, checks for index inconsistencies across the shards for sharded collections, and the command serverStatus, when run on the config server primary, returns the field shardedIndexConsistency field to report on the number of sharded collections with index inconsistencies.

If shardedIndexConsistency reports any index inconsistencies, you can run the following pipeline for your sharded collections until you find the inconsistencies.

Note

The following pipeline is for MongoDB 4.2.4 and above.

  1. Define the following aggregation pipeline:

    const pipeline = [
    // Get indexes and the shards that they belong to.
    {$indexStats: {}},
    // Attach a list of all shards which reported indexes to each document from $indexStats.
    {$group: {_id: null, indexDoc: {$push: "$$ROOT"}, allShards: {$addToSet: "$shard"}}},
    // Unwind the generated array back into an array of index documents.
    {$unwind: "$indexDoc"},
    // Group by index name.
    {
    $group: {
    "_id": "$indexDoc.name",
    "shards": {$push: "$indexDoc.shard"},
    // Convert each index specification into an array of its properties
    // that can be compared using set operators.
    "specs": {$push: {$objectToArray: {$ifNull: ["$indexDoc.spec", {}]}}},
    "allShards": {$first: "$allShards"}
    }
    },
    // Compute which indexes are not present on all targeted shards and
    // which index specification properties aren't the same across all shards.
    {
    $project: {
    missingFromShards: {$setDifference: ["$allShards", "$shards"]},
    inconsistentProperties: {
    $setDifference: [
    {$reduce: {
    input: "$specs",
    initialValue: {$arrayElemAt: ["$specs", 0]},
    in: {$setUnion: ["$$value", "$$this"]}}},
    {$reduce: {
    input: "$specs",
    initialValue: {$arrayElemAt: ["$specs", 0]},
    in: {$setIntersection: ["$$value", "$$this"]}}}
    ]
    }
    }
    },
    // Only return output that indicates an index was inconsistent, i.e. either a shard was missing
    // an index or a property on at least one shard was not the same on all others.
    {
    $match: {
    $expr:
    {$or: [
    {$gt: [{$size: "$missingFromShards"}, 0]},
    {$gt: [{$size: "$inconsistentProperties"}, 0]},
    ]
    }
    }
    },
    // Output relevant fields.
    {$project: {_id: 0, indexName: "$$ROOT._id", inconsistentProperties: 1, missingFromShards: 1}}
    ];
  2. Run the aggregation pipeline for the sharded collection to test. For example, to test if the sharded collection test.reviews has inconsistent indexes across its associated shards:

    db.getSiblingDB("test").reviews.aggregate(pipeline)

    If the collection has inconsistent indexes, the aggregation for that collection returns details regarding the inconsistent indexes:

    { "missingFromShards" : [ "shardB" ], "inconsistentProperties" : [ ], "indexName" : "page_1_score_1" }
    { "missingFromShards" : [ ], "inconsistentProperties" : [ { "k" : "expireAfterSeconds", "v" : 60 }, { "k" : "expireAfterSeconds", "v" : 600 } ], "indexName" : "reviewDt_1" }

    The returned documents indicate two inconsistencies for the sharded collection test.reviews:

    1. An index named page_1_score_1 is missing from the collection on shardB.

    2. An index named reviewDt_1 has inconsistent properties across the collection's shards, specifically, the expireAfterSeconds properties differ.

To resolve the inconsistency where an index is missing from the collection on a particular shard(s),

You can either:

To resolve where the index properties differ across the shards,

Drop the incorrect index from the collection on the affected shard(s) and rebuild the index. To rebuild the index, you can either:

Alternatively, if the inconsistency is the expireAfterSeconds property, you can run the collMod command to update the number of seconds instead of dropping and rebuilding the index.

←  Index IntersectionMeasure Index Use →
Give Feedback
© 2022 MongoDB, Inc.

About

  • Careers
  • Investor Relations
  • Legal Notices
  • Privacy Notices
  • Security Information
  • Trust Center
© 2022 MongoDB, Inc.