[daamien]

Temporary index and Hypothetical index are two different things.

Temporary index is a real index that's usually dynamically created by the optimizer to speed up a query. But it's kind of useless because most of the time the cost of creating a one-time index exceed the cost of not using it. Afaik only DB2 as this feature

http://www.centerfieldtechnology.com/PDFs/DB2%20Temp%20Index...

An hypotetical index is just a way to "trick" the planner into thinking that there's an index and see what the query plan would be if that index really existed. The cost of creating this is close to zero.