I first tried asking this question in the ArcGIS forums but as is common I got a total of 0 responses. I'm willing to accept this may be due to a lack of knowledge on the subject, but I'm optimistic. I will cross-pollinate responses or just delete the ArcGIS post if it remains untouched.
I've just started using Query Layers for read-only maps served through ArcGIS Server (10 SP2) reading data from Oracle 10g. I love it! I've always felt oddly uneasy about SDE and this seems a good sidestepping opportunity.
In my tests performance looks good but I would like to see some documentation (ideally) or other user feedback on Query Layer performance on live services or large datasets and how it compares to other DBMS options (SDE, SDE DC).
The only docs I've seen make a big deal out of the data being queried each time a user action occurs, but I would expect this to happen in all types of connections. Does SDE do some magical caching that a Query Layer doesn't? Does it better handle concurrent users in some way?
If anything I would expect Query Layers to improve performance at runtime (lower overhead) and reduce development / setup time (fewer SDE configuration headaches).
Any experience or informed thoughts on this would be very much appreciated. Please assume I am not concerned with versioning / multi-user editing, or any of SDE's other non performance-related benefits.