I have to say that I wasn't pleased at all toe see how the Galaxy C9 Pro performs in low-light situations. I don't know which sensor was used for the back camera, but even with an f/1.9 aperture lens, the low-light performance just didn't cut it for me. I've seen quite a few camera samples taken in various ISO sensitivities. The image quality start to suffer badly from ISO 800 and above. At ISO 1600 the image is very patchy with lots of details smearing all over the image. To get a reasonable image, you need to have a good amount of light in the image, so the camera won't shoot at a very high ISO speed.
This is certainly the limitation of the sensor, not the lens. The lens is very fast, but the sensor's small pixels just limit its low-light capabilities. When I see a 16MP sensor in a mobile phone camera, my expectations immoderately drop, especially when dealing with mid-range phones. With high-end devices you usually get high-end sensors that utilize advanced sensor technologies that can produce good photos even with a less than ideal resolution. Some of those sensors are usually larger and therefore boast large pixel than normal. Of course I would be happy to know what is the exact sensor model just for reference, but the camera samples speak for themselves.
The Samsung Galaxy C9 Pro also lacks an optical image stabilization (OIS). An OIS could have helped here, because the camera could have been programmed to take advantage of it and shoot at a slower shutter speed while maintaining a low ISO sensitivity. The phone doe have a Low-light mode that help a bit, but the image is still very noisy.
The thing is that you really can't do much to improve the low-light performance. A good thing was to have an option to shoot at a slower shutter speed, but the Pro mode lacks that option. You don't have any option to manually control the shutter speed directly and you need to rely on the camera to make the best decision. Even if you put the phone on a stable surface or mount it on a tripod, it doesn't matter, because the camera doesn't know that it's on a stable surface and won't lower the shutter speed to let more light in.
This is one of the limitations of the C9 Pro primary camera when it comes to low-light photography. It's certainly a disadvantage, and I personally wouldn't recommend this phone for photography enthusiasts, especially for those who are looking for a phone with a primary camera that can take good pictures in low light situations.