Time-series collections in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Querying time-series data with a time filter

Given a MongoDB time-series collection named sensorData with documents containing timestamp and temperature fields, what will be the output of this query?

db.sensorData.find({ timestamp: { $gte: ISODate('2024-01-01T00:00:00Z'), $lt: ISODate('2024-01-02T00:00:00Z') } })

Assume the collection has 3 documents with timestamps on 2023-12-31, 2024-01-01, and 2024-01-02.

MongoDB

db.sensorData.find({ timestamp: { $gte: ISODate('2024-01-01T00:00:00Z'), $lt: ISODate('2024-01-02T00:00:00Z') } })

A Returns no documents

B Returns documents with timestamps on 2024-01-01 and 2024-01-02

C Returns only the document with timestamp on 2024-01-01

D Returns all three documents

💡 Hint

Think about the meaning of $gte and $lt in the time range filter.

The query filters documents where timestamp is greater than or equal to 2024-01-01 and less than 2024-01-02. So only documents on 2024-01-01 are included, excluding 2024-01-02.

🧠 Conceptual

intermediate

Understanding time-series collection schema requirements

Which of the following is required when creating a time-series collection in MongoDB?

MongoDB

db.createCollection('sensorData', { timeseries: { timeField: 'timestamp', metaField: 'deviceId' } })

A The <code>timeField</code> must be specified and contain date values

B The <code>metaField</code> must be specified and contain numeric values

C The collection must have an index on the <code>timeField</code> before creation

D The collection must be capped

💡 Hint

Think about what MongoDB uses to organize time-series data.

The timeField is mandatory and must contain date/time values to store time-series data properly. The metaField is optional and can contain any metadata.

📝 Syntax

advanced

Identify the syntax error in time-series collection creation

Which option contains a syntax error when creating a time-series collection named weatherData with recordedAt as the time field?

A db.createCollection('weatherData', { timeseries: { timeField: 'recordedAt', metaField: 123 } })

B db.createCollection('weatherData', { timeseries: { timeField: 'recordedAt' } })

C db.createCollection('weatherData', { timeseries: { timeField: 'recordedAt', metaField: 'location' } })

D db.createCollection('weatherData', { timeseries: { timeField: 'recordedAt', granularity: 'minutes' } })

💡 Hint

Check the type of the metaField value.

The metaField must be a string naming the metadata field. Using a number like 123 causes a syntax error.

❓ optimization

advanced

Optimizing queries on time-series collections

You want to optimize queries filtering by device ID and time range on a time-series collection sensorReadings. Which index strategy is best?

A No indexes needed because time-series collections are always optimized

B Create a compound index on <code>{ deviceId: 1, timestamp: 1 }</code>

C Create a single index on <code>timestamp</code> only

D Create a single index on <code>deviceId</code> only

💡 Hint

Think about filtering by both device and time.

A compound index on deviceId and timestamp helps queries filter efficiently by both fields.

🔧 Debug

expert

Why does this time-series query return no results?

You run this query on a time-series collection tempLogs:

db.tempLogs.find({ timestamp: { $gt: new Date('2024-01-01T00:00:00Z') } })

But it returns no documents, even though you know data exists after that date. What is the most likely reason?

MongoDB

db.tempLogs.find({ timestamp: { $gt: new Date('2024-01-01T00:00:00Z') } })

A The collection is empty

B The query syntax is invalid and causes an error

C The dates in the collection are stored as strings, not dates

D The <code>timestamp</code> field is not the timeField defined in the collection

💡 Hint

Check the collection's timeField configuration.

Time-series collections only index and query efficiently on the defined timeField. If timestamp is not the timeField, queries on it return no results.