Users started complaining about spotty application performance; sometimes it was good, sometimes it was horrible. The first thing came to my mind was some kind of database bottleneck. In my experience, CPU and I/O contention are usually the culprits but one time (not this time) it actually was a network problem. Here's how I broke it down...

A team member thought we should add an index on a 90 million row table to improve performance. The field on which he wanted to create this index had only four possible values. To which I replied that an index on a low cardinality field wasn't really going to help anything. My boss then asked me why wouldn't it help? I sputtered around for a response but ended up telling him that I'd get back to him with a reasonable explanation.

In oracle, you can copy data between linked databases via normal SQL.

insert into table select * from table@db_link

The databases can even be on different physical machines.