What Freakin Database Should I Use? (SQL vs other variants, “NoSQL”)
Reddit tries to pre-compute everything possible to minimize user waiting time.
However, famous database guys say “no, you're doing it wrong”. here
https://wiki.postgresql.org/wiki/Why_PostgreSQL_Instead_of_MySQL:_Comparing_Reliability_and_Speed_in_2007
http://stackoverflow.com/questions/110927/would-you-recommend-postgresql-over-mysql