Multiple query vs Single (multiple has many joins) - sql

Question

Recently stumbled on this situation. Doing both queries might be "light" in my situation, I just want to know when it comes to big dataset on what is better. Better in overall (performance, speed, etc etc).
Currently I do single queries of 2 1:N (has-many) relationship and reduce/transform the data in the application.
It looks like this transformed/reduced:
[
'field' => 'value',
'hasMany-1' => [],
'hasMany-2' => []
]
I'm actually somehow tempted to just do separate queries as it eliminates the pain of reducing it if I had more than 2 hasMany queries and is more quite readable but code currently works so I'll maybe just do it next time.
Is the compromise worth it? Again, in my situation it might be very "light" as I only have few rows (< 100) and structure is not complex as it is on early stage yet.
But asked in case I stumble upon this next time and when dataset grows larger.
** EDIT **
So the has-many relationship I'm talking about are: A customer has-many phones and pets.
My current query returns me this result (simplified):
customer_id | pet_name | phone
1 | john | 1234
1 | john | 5678
2 | jane | 1357
2 | jane | 2468
2 | joe | 1357
2 | joe | 2468
I think my query is fine. It seems logical for some rows to repeat because the other field has different value.

Answer 1

In general, you should issue a single query and let the optimizer do the work for you. At the very least, this saves multiple round-trips to the database and query compilation.
There are cases where multiple queries can have better performance, but I think it is better to start with a single query.
You have a particular issue regarding joins along multiple many-to-many dimensions. There is no need to do the joins "generally" and then "reduce" the results. There are more efficient methods.
I would suggest that you ask another question. Provide sample data, desired results, and an explanation of the logic you are attempting. You may be able to learn a more efficient way to write a single query.

Answer 2

You did not describe your table structures so i assume few things.
If you want to have pets and phones as one row do:
select c.customer_id, c.name,
array_to_string((array_agg(p.pet_name)),',') pet_names
array_to_string((array_agg(ph.phone)),',') phones
from customer c, pet p, phones ph
where p.customer_id=1
and p.customer_id=c.customer_id
and ph.customer_id=c.customer_id
group by c.customer_id, c.name
If you want to have row per pet_name with all possible phone numbers:
select c.customer_id, c.name,
p.pet_name
array_to_string((array_agg(ph.phone)),',') phones
from customer c, pet p, phones ph
where p.customer_id=1
and p.customer_id=c.customer_id
and ph.customer_id=c.customer_id
group by c.customer_id, c.name, p.pet_name
If we talk about performance it will be faster to do 2 queries to pets and phones separetly by customer_id. But until you have milions of rows it is not so important.
Of course you should have indexes on customer_id.