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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
The simplicity of tree-navigation in "closure" is pretty amazing for how simple it is: https://github.com/nalgeon/sqlean/issues/27
Given that SQLite supports huge sizes for text, you could almost use the above as a way to create an outliner, with everything stored in the database itself ...
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