I have actually heard of Kx and Kdb+ and Q and have looked into those products. You're right that I should have mentioned them, because they are important in that space, and in many ways they are miles ahead. The state of time series query languages is quite poor (outside of Influx's effort), so there is a lot to learn from Kdb+ as well.

I was however focusing on recent Open Source efforts and on the general approaches. Hence, I didn't really discuss in detail my own tsdb.

I also find virtually everything to do with K and Kdb to be be simultaneously impressive and utterly unfathomable:

http://code.kx.com/wsvn/code/kx/kdb%2B/s.k

This is a cheap jab to make, but it makes these systems pretty impenetrable from a source level.

Possibly because they cost an arm and a leg (or at least that's the perception) and are therefore out of reach of most firms, apart from large utilities and hedge funds, and the language looks like line noise.

Yes, I know there is a free version, but limited to 32-bit only (and probably non-commercial?).

EDIT: 32-bit version can be used commercially.