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.
QuestDB and InfluxDB are both open-source time-series databases, but with different architectural approaches. QuestDB uses a columnar storage model with SQL support, while InfluxDB uses its own query language (InfluxQL/Flux) and a time-series-specific storage engine.