This doesnt quite ring true to me. I'm assuming you're talking about performance...? However I'm interested to know how you correlate user data back to a user without using the session_key as the identifier of the user...

I have a setup where a user may store personal data on website (for example a playlist). I use an RDMBS as the storage mechanism as it is robust, relatively fast and i can port my SQL92 complient application to another RDMBS relatively painlessley. For the user to edit that bit of data (for me to retrieve the correct data) I correlate the session_id to a user_id via sql...

OT to the original post, i'm interest in what mechanism you use, or would recommend..