I like leaving compute_query_id=auto instead of overwriting it to "on", even when queryIsWanted() is called, as I told upthread. Then we can decide that query id computation is necessary when compute_query_id=auto and the special flag is enabled by queryIsWanted(). Of course as you and Magnus discussed upthread, the issue in EXEC_BACKEND case should be fixed, maybe by using save_backend_variables() or something, though.
If we do this, compute_query_id=auto seems to be similar to huge_pages=try. When huge_pages=try, whether huge pages is actually used is defined depending outside PostgreSQL (i.e, OS setting in this case). Neither pg_settings.setting nor souce are not changed in that case.
I'm fine with that, but a lot of people complained that it wasn't good because you don't really know if it's actually on or not. I personally don't think that it's an issue, because what user want is to automagumically do what they want, not check how the magic happened, and if they want a third party implementation then the module can error out if the setting is on, so the burden will only be for those users, and handled by the third party module author.
