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Set Granularity for Time Series Data

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  • Retrieve the granularity of a Time Series Collection
  • Change the granularity of a Time Series Collection
Note

You must be running MongoDB 5.0.1 or later in order to change a time series collection's granularity after the collection has been created. See MongoDB 5.0 known issues.

When you create a time series collection, set the granularity to the value that is the closest match to the time span between consecutive incoming measurements that have the same unique value for the metaField field:

db.createCollection(
"weather24h",
{
timeseries: {
timeField: "timestamp",
metaField: "metadata",
granularity: "minutes"
},
expireAfterSeconds: 86400
}
)

Setting the granularity parameter accurately improves performance by optimizing how data in the time series collection is stored internally.

To set the parameter accurately, choose a granularity value that is closest to the ingestion rate for a unique data source as specified by the value for the metaField field.

For example, if your metaField data identifies weather sensors and you ingest data from each individual sensor once every 5 minutes, you should choose "minutes". Even if you have thousands of sensors and the data coming in from different sensors is only seconds apart, the granularity should still be based on the ingestion rate for one sensor that is uniquely identified by its metadata.

In the following table, you can see the max time span of data that is stored together for each granularity value:

granularity
Covered Time Span
"seconds" (default)
one hour
"minutes"
24 hours
"hours"
30 days
Tip
See also:

To retrieve the current value of granularity, use the listCollections command:

db.runCommand( { listCollections: 1 } )

The result document contains a document for the time series collection which contains the options.timeseries.granularity field.

{
cursor: {
id: <number>,
ns: 'test.$cmd.listCollections',
firstBatch: [
{
name: <string>,
type: 'timeseries',
options: {
expireAfterSeconds: <number>,
timeseries: {
timeField: <string>,
metaField: <string>,
granularity: <string>,
bucketMaxSpanSeconds: <number>
}
},
...
},
...
]
}
}

To change the granularity parameter value, issue the following collMod command:

db.runCommand({
collMod: "weather24h",
timeseries: { granularity: "hours" }
})

Once the granularity is set it can only be increased by one level at a time. From "seconds" to "minutes" or from "minutes" to "hours". Other changes are not allowed. If you need to change the granularity from "seconds" to "hours", first increase the granularity to "minutes" and then to "hours".

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