A big advantage of Oracle (and perhaps Postgres, but I'm not qualified to speak about it) over MySQL is that it scales better. Oracle is huge and somewhat bloated, but it is really intended to hold up even when stressed out. I could be wrong on this, but it doesn't look like MySQL even has a library cache (see here and here), which databases like Oracle use to cache parsed queries and their execution plans. A typical web application is going to generate many, many of the same queries (with different values plugged into the WHERE clause) over and over again. An application using Oracle bind variables (and shame on you if you aren't) will mean that the overhead associated with parsing queries is simply not there. Tweak the Oracle shared pool size so that it'll hold all the needed queries in cache (and use bind variables), and parsing overhead goes away. The queries will only be parsed the first time they're executed.
For applications without much load, the raw speed of
its "lightweightness" makes MySQL look like a screamer compared to
big, bloated Oracle. But increase the load, and Oracle simply
ramps up while MySQL struggles under its own lack of sophistication.
I believe similar results were shown by the big PostgreSQL versus MySQL
web benchmark that was done a year or two ago. Under small loads,
MySQL won. As traffic increased, PostgreSQL coped much better. Based on
that, it looks like Slashdot needs a lot of hardware to support
software that's not as fancy as it should be.
