In DTPO, where is the preference to “update indexed databases on open” ? Is there one?
No, there is no preference for this. The script found here may assist you: Updating indexed groups across many databases
Thanks … but how is using this any easier than just selecting the “Update Indexed Items” selection under the “File” menu ?
The Update Indexed Items menu only acts on the current selection, while the script updates all indexed items across all open databases.
Additionally, with this script, you can automatize things conveniently using launchd.
In my case, modified the script a little bit, so it can run in background without the dialog box. Also, I let it only ran on UUID of databases where I know indexed files are present (mostly for performance reasons).
Then, I created a launchd entry which runs my script like once per hour all day in the background. So I always have current data in my DTPO (updated at least 59 minutes ago.)
I have around 7000 indexed files with around 30 million words and the script just takes a couple of seconds to complete. If you have more indexed files, it might be advisable to adjust the frequency (once per day, whatever you need.)
That being said, I try to keep most documents inside my DTPO database. I only use indexed files for things which make no sense to keep inside the database, as they need to keep their folder structure for external applications and mostly only want them inside DTPO for search purposes (in my case, that would be source files and some programming-related projects.)
Show-off. 
Haha 
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Well, using something like [1], you don’t really need advanced knowledge of these things. It’s mostly drag and drop. (And I hope this does not count as advertisement, as I’m not affiliated with the developers of that tool.)
Anyway, if someone needs assistance with setting something like this up, I’d of course be open to questions 
Though, it might be delayed as I’ll go on winter vacation from tomorrow. 