I do not claim expertise here, but it would seem like microservices would add significant performance costs. Stitching together a bunch of results from different microservices is going to be a LOT more expensive than running a query with joins.
But to speak directly to your concern, you have to think about service boundaries and granularity correctly. Nobody is saying make a microservice out of every conceivable table. Think about the bigger picture, at a systems level. Wherever you can draw boxes you might have a service boundary.
Why would you need to join payment data to session and login data?
Do you need to compare employee roles and ACLs against product shipping data?
These things belong in different systems. If you keep them in the same monolith, there's the danger that people will write code that intertwines the model in ways it shouldn't. Deploying and ownership become hard problems.
The goal is to keep things that are highly functionally related together in a microservice and expose an API where the different microservices in your ecosystem are required to interact. (Eg, your employees will login.)
When the data analytics folks want to do advanced reporting on the joins of these systems (typically offline behavior), you can expose a feed that exports your data. But don't expose an internal view of it to them or they'll find ways of turning you into a monolith.
Also then what also happens is microservices are created using different languages which in turn adds so much complexity to understand what is going on on the whole big picture level.
And code gets repeated a lot more. If there is change in a microservices or update everyone will need to figure out what services depend on and how they will have to adapt. With monolith you can just use your IDE to see what will break if you make a change. So much repeated business logic. Creating a new feature involves having to have many meetings to figure out what services in which way have to be updated.
It is crazy mess in my opinion.
I have been with a company that had monolith application which they split up to more than 15 services (some python, some js, Scala, Java, etc...). Monolith still is used for some parts that are not migrated. I was working on single service having no idea how the whole system worked together. Then I had to do something in the old parts and I very quickly got an understanding how everything works together.
This is what people mean when they say "distributed monolith" vs. microservices.
And holy fuck is debugging that stuff difficult. HUUUUUGE waste of time, but management looooooooooves their blasted microservices...
With microservices, without a good documentation how it connects, it's going to leave a very bad impression.
It is still nowhere close to ability to jumping around with IDE.
It might be in a different language, different design patterns and to get to the details you have to check out that project anyway because you can't document absolutely everything out of code base. And if you do you will end up with multiple sources of truth.
It is so much more likely that for every little issue which you otherwise might be able to find an answer to yourself very easily you will have to contact the team owning that microservices.
It is not only mentally exhausting. It is time consuming, it requires so much back and forth. It creates so much dependence on other people because figuring out how things are related is so much more difficult.
Sometimes I have 8 or more different IDE windows open to understand what is going on.
This is the hardest part.. I'd argue that this is almost impossible to do correctly without significant domain modeling experience.. also microservices by nature make this hard to refactor these boundaries (compared to monoliths where you'd get compile time feedback)
I prefer to make a structured monolith first (basically multiple services with isolated data that are squished together into a single deployable) and pull them out only if I really need to... Also helps with keeping ms sprawl under control
That's what SOA and microservices is supposed to solve.
At that scale you do reporting from a purpose-built service.
Allegedly.
There's going to be a relationship between data in your services, but it shouldn't be directly referential.