Most (all?) RDBMS's have something like a varchar(max) which accepts reasonably large strings.
> you have to create indexes and fus with all sorts of relational nonsense
No, you don't.
> its going to read the entire table into memory and re-write it back to disk
Probably not, if you're adding a nullable column.
That's not "cheating", it's literally the only relevant factor.
Who would win in a fight: a superintelligent ninja cat with laser nunchucks and body armor, or a dog?
While standard SQL may have this problem, most concrete RDBMS do not, because they support a simple length-unspecified string type (or, in the case of SQLite, treat column types as documentation and mild hints rather than actual rules.)
> you have to create indexes
Absolutely not. You can, for efficiency, but (ironically, unlike some noSQL systems that require explicit indexes to support queries) you can query and join tables of relational databases without defining explicit indexes. This may, for some queries, be extraordinarily inefficient (it may not, if you've defined the schema well, because many RDBMSs implicitly create indexes based on schema features like PKs and unique constraints), but you can do it.
imagine you just received two TB data sets, and you'd like to join them and run a process. In hive, you create the schema (pretty simple) and join them. queries may take a few minutes but you get to doing the work you intended all along.
In RDBMS, you have to fus with its schema, "you said 125 characters for that string field and you just received 130, FAIL". Its a STRING JUST DEAL WITH IT. you have to create indexes and fus with all sorts of relational nonsense when it has little to do with the work you are trying to accomplish.
take your billion row database, and now you need to add a new field. its going to read the entire table into memory and re-write it back to disk, meanwhile you aren't doing the work you were trying to accomplish.
relational databases are great for small static data, but when you deal with new large messy data every day, they sap all your time.