Is using timestamp as primary key good idea - mysql/mariadb?
IMO it's bad because of several reasons. First, blindly assigning a generated p.k. for every relation is bad practice. At least try to identify what candidate keys there are in each relation. A surregate p.k. may be the smartest choice in some situations, but it is definitely not so in all possible situations. Good properties for candidate keys are:


Unique
Irreducible
Non-complex
Stable
Familiar


Often there is a conflict between these criteria and you have to sacrifice one for another.

Second, if the best choice is a surrogate identifier I'll say that timestamp is a poor choice. Especially in MySQL/MariaDB before (MySQL) 5.6.4 where the resolution is seconds. Using a timestamp as a p.k in these DBMS means that you cannot insert more than one row / second (as long as the timestamp should reflect the time of insert).