Time Series Indexes
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Indexes on time series collections generally behave like indexes on regular collections, but with several additional considerations and limitations.
If there are secondary indexes on time
series collections and you need to
downgrade the feature compatibility version (fCV), you must first drop
any secondary indexes that are incompatible with the downgraded fCV.
For more information, see setFeatureCompatibilityVersion.
Starting in version 6.0, you can add a secondary index to any field in a time series collection. MongoDB indexes time series collections by buckets of documents as opposed to individual documents. Time series buckets contain documents with shared metaField values, ordered by timeField values that are close together. MongoDB indexes the minimum and maximum values of all fields, except the metaField. Indexing buckets instead of individual documents reduces index size and improves query efficiency.
Clustered Collection
By default, MongoDB clusters time series collections based on bucket time.
Compound Indexes
New in version 6.3.
Starting in MongoDB 6.3, MongoDB creates a default compound index on both the metaField and timeField of a time series collection. MongoDB uses this index to improve query performance and speed.
You can add a compound index on the
timeField, metaField, or measurement fields.
Partial Indexes
New in version 6.0.
Starting in MongoDB 6.0, you can use the $or, $in, and
$geoWithin operators with partial indexes on a time series collection.
You cannot create partial indexes on the metaField and timeField.
TTL Indexes
New in version 6.3.
Starting in MongoDB 6.3, you can create a TTL
index with a partialFilterExpression that relies only on the
metaField. In versions prior to 6.3, you can only create TTL indexes
based on the expireAfterSeconds parameter.
If your time series collection doesn't use the expireAfterSeconds
option to expire documents, creating a partial TTL index sets an
expiration time for matching documents only. If the collection uses
expireAfterSeconds for all documents, you can use a partial TTL
index to expire matching documents sooner.
Prohibited Indexes
MongoDB does not allow the following index types on time series collections:
You cannot create sparse indexes on the metaField.
Indexing Best Practices
Use the metaField index for filtering and equality.
Use the timeField and other indexed fields for range queries.
General indexing strategies also apply to time series collections. For more information, see Indexing Strategies.
For more information and examples, see Add Secondary Indexes to Time Series Collections.