Microsoft SQL Server
...and almost all other relational databases engines and many NoSQL engine. Many of these engines are hybrids between C and C++. Others are pure C.
Even if they were "from scratch", C (possibly C++ at higher level) would still be the best programming language for them. DB engines - just like OS kernels – are environments that require extreme clarity about what code is doing. "Mysterious" programmatic abstractions, in contrast to internal and external APIs, modularity and modularity which are widely available and widely used with C, make it harder to debug.
DB engines can be very difficult to tune internally. The "inner loop" may exist in different parts depending on which queries are being run. You will need to tune performance of many parts of the db engines.