I have an app using an Oracle 11g database. I have a fairly large table (~50k rows) which I query thus:

SELECT omg, ponies FROM table WHERE x = 4

Field x was not indexed, I discovered. This query happens a lot, but the thing is that the performance wasn't too bad. Adding an index on x did make the queries approximately twice as fast, which is far less than I expected. On, say, MySQL, it would've made the query ten times faster, at the very least.

I'm suspecting Oracle adds some kind of automatic index when it detects that I query a non-indexed field often. Am I correct? I can find nothing even implying this in the docs.