Why do some sites use several different DBMS?

Question

Database

Well, this question has been of interest for a long time. I also read about VK that they use two DBMS: their self-written DBMS, and mysql. Now I was also told to do one task, the person said that it is desirable for users to be stored in mysql, and what information will need to be selected in mongo.
Question: why in all cases it is impossible to use one subd? To store users in one table, those data with what it is necessary to work - in another. Why split into subd ?, it's inconvenient to work as a programmer with this

Answer 1

It is convenient to cut bread with a knife, to slurp cabbage soup with a spoon,
why can't you use one tool?
Each has its own advantages, if you design poorly, it will not be convenient to work with them.

Answer 2

There is a myth that Mongo is faster and is used as a cache or JSON storage. In general, you should use PostgreSQL and not complicate your life. Mongo has its own, limited, scope, and I do not see the advantages of imperative programming over declarative programming. Let me remind you that writing views in SQL is declarative programming and, in general, relational databases are completely mathematically justified. And the pipelined query language in Mongo is such a dance created during the development of Mongo