TimescaleDB and QuestDB are both SQL-based time-series databases. TimescaleDB extends PostgreSQL, offering the full Postgres ecosystem and ecosystem compatibility. QuestDB is purpose-built for time-series with a focus on extreme ingestion performance and SQL simplicity.
TimescaleDB and InfluxDB are both time-series databases but with different architectural approaches. TimescaleDB extends PostgreSQL, offering full SQL compatibility and the Postgres ecosystem. InfluxDB uses a custom query language (InfluxQL/Flux) and is purpose-built for time-series from the ground up.