HT performance grew from generation to generation. In many cases a HT "core" has 80% of the performance of a "normal" core. A current 2-core+HT i3 CPU is almost on par with a 4-core i5 (the i3 has faster cores but less L3 cache).
I presume a database must be memory intensive.
Mine is video encoding using FFMPEG (where I run more program threads than FFMPEG decides is optimal in order to make sure no cpu thread is idle).
I get at most 15% over hyperthreading off, and often about 5-10% less than hyperthreading off (For the whole process, beginning to end).
On real cores, it is close to being linear in the number of cores (not quite, but it is close).
For decades now we've had to suffer these weird internet arguments about whether hyperthreading is "real", as if there was a naturally-occuring correct number of frontends on a CPU. If someone wants to put more instruction decoders on their CPU there is no rational argument that is "wrong".