Amazon Timestream is a fast, scalable, and serverless time-series database service for IoT and other operational applications that makes it easy to store and analyze trillions of events per day up to 1,000 times faster and at as little as 1/10th thecost of relational databases1.Amazon Timestream saves you time and cost in managing the lifecycle of time series data, and its purpose-built query engine lets you access and analyze recent and historical data together with a single query1.Amazon Timestream has built-in time series analytics functions, helping you identify trends and patterns in near real time1.

The other options are not suitable for storing and analyzing trillions of events per day. Amazon Neptune is a graph database service that supports highly connected data sets. Amazon Forecast is a machine learning service that generates accurate forecasts based on historical data. Amazon DocumentDB (with MongoDB compatibility) is a document database service that supports MongoDB workloads.

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1: Time Series Database -- Amazon Timestream -- Amazon Web Services