It's not just complex databases, but any database that uses indexes (basically any production database). Deleting a record from a table can cause a re-index, w

It's not just complex databases, but any database that uses indexes (basically any production database). Deleting a record from a table can cause a re-index, which is very intensive. It's much easier to flag a column in a record as "deleted" or whatever, and then run a cleanup during off-peak hours. I'm sure there are clever ways around that with proper knowledgeable DBAs on your team, but as I'm a web dev for smaller audience projects, I don't touch solutions that require those types of optimizations that I'm sure Facebook has implemented.