Databend source reading

[JasonLi-cn]

I recently took some brief notes while studying the Databend source code. Because Databend is updated so quickly, the code mentioned in the notes may be out of date. So I created some different branches to be consistent with the Databend version.
I'll try to make my notes more detailed, but my energy is limited, may not do perfect. I hope it will be helpful to other Databend newer. 😃

https://github.com/JasonLi-cn/databend-comment