Disadvantage of SQL Joins

Can anybody please tell me whether there are any disadvantage of SQL
joins in terms of space and time and how postgres has implemented SQL
joins? I am in the impression that SQL join takes more time for
execution as well as space because database internally builds Cartesian
product and then evaluates for condition. Recently I normalized by DB
and changed my application Data model which resulted in writing lots of
inner join queries. Later I found that DB server is consuming more
memory. And once my Database also crashed. I am assuming that because of
normalization and inner joins, my DB crashed.
Can anybody please give a thought into my assumption?

- R