# Reference

- [Single-stage query engine (v1)](/release-1.2.0/reference/single-stage-engine.md): An overview of the single-stage query engine.
- [Multi-stage query engine (v2)](/release-1.2.0/reference/multi-stage-engine.md): An overview of the multi-stage query engine.
- [Troubleshooting](/release-1.2.0/reference/troubleshooting.md)
- [Troubleshoot issues with the multi-stage query engine (v2)](/release-1.2.0/reference/troubleshooting/troubleshoot-multi-stage-query-engine.md): Troubleshoot issues with the multi-stage query engine (v2).
- [Troubleshoot issues with ZooKeeper znodes](/release-1.2.0/reference/troubleshooting/troubleshoot-zookeeper.md): Troubleshoot issues with Zookeeper znodes.