As an amusing anecdote, my friend was working with a _really_ slow query (it took literally days to run a query), and their consult/software vendor/whatnot that

As an amusing anecdote, my friend was working with a _really_ slow query (it took literally days to run a query), and their consult/software vendor/whatnot that had written the query kept telling that your data is so large/complex that you really need to look into big data systems like Hadoop. Turns out the SQL query was running on for-loops. With some tinkering my friend got rid of the loops, and after that the query run in less than three minutes. No more talking about Hadoop there.