But is that really important? Consider that the UI has to make a network call to the API; that's pretty big (order of magnitude of milliseconds). Databases are optimized to keep things in memory and execute reads very, very quickly (eg. SQL Server loads and keeps everything in RAM and consumes almost all your free RAM if it can).
In theory, you are correct. However, there are a few flaws with this rationale:
From what you stated, it's unclear if you actually tested / profiled your app. In other words, do you actually know that the network transfers from the app to the API are the slowest component? Because that is intuitive, it is easy to assume that it is. However, when discussing performance, you should never assume. At my employer, I am the performance lead. When I first joined, people kept talking about CDN's, replication, etc. based on intuition about what the bottlenecks must be. Turns out, our biggest performance problems were poorly performing database queries.
You are saying that because databases are good at retrieving data, that the database is necessarily running at peak performance, is being used optimally, and there is nothing that can be done to improve it. In other words, databases are designed to be fast, so I should never have to worry about it. Another dangerous line of thinking. That's like saying a car is meant to move quickly, so I don't need to change the oil.
This way of thinking assumes a single process at a time, or put another way, no concurrency. It assumes that one request cannot influence another request's performance. Resources are shared, such as disk I/O, network bandwidth, connection pools, memory, CPU cycles, etc. Therefore, reducing one database call's use of a shared resource can prevent it from causing other requests to slow down. When I first joined my current employer, management believed that tuning a 3 second database query was a waste of time. 3 seconds is so little, why waste time on it? Wouldn't we be better off with a CDN or compression or something else? But if I can make a 3 second query run in 1 second, say by adding an index, that is 2/3 less blocking, 2/3 less time spent occupying a thread, and more importantly, less data read from disk, which means less data flushed out of the in-RAM cache.
There is a common conception that software performance is simply about speed.
From a purely speed perspective, you are right. A system is only as fast as its slowest component. If you have profiled your code and found that the Internet is the slowest component, then everything else is obviously not the slowest part.
However, given the above, I hope you can see how resource contention, lack of indexing, poorly written code, etc. can create surprising differences in performance.
One last thing. You mentioned that a database call should be cheap compared to a network call from the app to the API. But you also mentioned that the app and the API servers are in the same LAN. Therefore, aren't both of them comparable as network calls? In other words, why are you assuming that the API transfer is orders of magnitude slower than the database transfer when they both have the same available bandwidth? Of course the protocols and data structures are different, I get that, but I dispute the assumption that they are orders of magnitude different.
Where it gets murkey
This whole question is about "multiple" versus "single" database calls. But it's unclear how many are multiple. Because of what I said above, as a general rule of thumb, I recommend making as few database calls as necessary. But that is only a rule of thumb.
Here is why:
- Databases are great at reading data. They are storage engines. However, your business logic lives in your application. If you make a rule that every API call results in exactly one database call, then your business logic may end up in the database. Maybe that is ok. A lot of systems do that. But some don't. It's about flexibility.
- Sometimes to achieve good decoupling, you want to have 2 database calls separated. For example, perhaps every HTTP request is routed through a generic security filter which validates from the DB that the user has the right access rights. If they do, proceed to execute the appropriate function for that URL. That function may interact with the database.
- Calling the database in a loop. This is why I asked how many is multiple. In the example above, you would have 2 database calls. 2 is fine. 3 may be fine. N is not fine. If you call the database in a loop, you have now made performance linear, which means it will take longer the more that is in the loop's input. So categorically saying that the API network time is the slowest completely overlooks anomalies like 1% of your traffic taking a long time due to a not-yet-discovered loop that calls the database 10,000 times.
- Sometimes there are things your app is better at, like some complex calculations. You may need to read some data from the database, do some calculations, then based on the results, pass a parameter to a second database call (maybe to write some results). If you combine those into a single call (like a stored procedure) just for the sake of only calling the database once, you have forced yourself to use the database for something which the app server might be better at.
- Load balancing: You have 1 database (presumably) and multiple load balanced application servers. Therefore, the more work the app does and the less the database does, the easier it is to scale because it's generally easier to add an app server than setup database replication. Based on the previous bullet point, it may make sense to run a SQL query, then do all the calculations in the application, which is distributed across multiple servers, and then write the results when finished. This could give better throughput (even if the overall transaction time is the same).
TLDR: Is it really significant to worry about multiple database calls when we are already making a network call over the LAN? If so, why?
Yes, but only to a certain extent. You should try to minimize the number of database calls when practical, but don't combine calls which have nothing to do with each other just for the sake of combining them. Also, avoid calling the database in a loop at all costs.