Hello, I'm Jaromir, one of the core engineers at QuestDB team. I just noticed this blog is trending! Andrei - the author - lives in Bulgaria and he is probably already sleeping. Happy to answer any question the blog left unanswered.
1. correctness: from small units tests to relatively complex integrations tests. they typically populate a test database and query it via various interfaces, such as REST or the Postgres protocol. we use Azure Pipelines to execute them - testing in MacoOS, Linux (both Intel and ARM) and Windows.
2. performance: we tend to use the TSBS project for most of our performance testing and profiling. fun fact: we actually had to patch it as the vanilla TSBS was a bottleneck in some tests. Sadly, the PR with the improvements is still not merged: https://github.com/timescale/tsbs/pull/186
edit: I thought I would link some of the more interesting tests: Since QuestDB supports the Postgres wire protocol we have to gracefully handle even various half-broken Postgres clients. But how do you write a test with mimicking a client generating invalid requests? No sane client will generate broken requests. So we use Wireshark to record network communication between the broken client and our server and then use this recorded communication in tests. Example: https://github.com/questdb/questdb/blob/3995c31210c70664d4b3...
