If you have worked only with MySQL till now and planning to shift you apps in production on Oracle then you may run into a few very nasty gotchas.
1. "30 character" limit: I had a long (19 characters) table name (required due to rails convention) and some pretty long column names (due to legacy schema thingy). I wrote the find methods and bang! An error which said Oracle doesn't support identifiers longer than 30 characters! Whoa!!! I still don't understand what could be the logic behind a 30 character limit!
2. Oracle doesn't have a 'limit' keyword. Yes you read it correctly ! You cannot say something like this "select * from < table > limit 10, 100"! Oracle doesn't support this at all. You would need sub selects and stuff
3. Some of the table names which are allowed in mysql are not in Oracle like "date"...if I am not mistaken
If you serialize a column and try to read the column if no data is there it returns a rather nasty error which says "ActiveRecord::SerializationTypeMismatch:"