unity-dash
What kind of items and which attributes are indexed? E. g. desktop/launchers, documents; what metadata?
Where's the index stored? sqlite, couchdb, …?
Which query language is used; what's the query like? How are results ranked/ordered? I'd like to see an example query.
I found the unity-place-python sample code and used that to create a lens to search your filesystem. It's currently kinda unstable, but you can search for files and you usually get results.
You can access the lens in the Unity Launcher. You can see it here where I'm searching for "lecture".
The magnifying glass in the bottom left corner is the FileSearch Lens. (The icon changes based on your icon theme.) The lens shows files and folders in two separate sections.
You can add it to your main dash (Ubuntu button) by setting ShowGlobal=true in /usr/share/unity/places/filesearch.place. Unfortunately, Unity doesn't currently have a way to customize what lenses are in the Dash.
The files and folders sections are combined in the dash under the heading "File Search".
When filesearch-lens first runs, it will create a config file ~/.filesearch.cfg. You can customize the directories to search, how deep to search, and what folders to ignore.
For example, here's my config:
[search]
#dirs = a comma separated list of folders to search
dirs = ~/data
#depth = how deep from / to search
depth = 5
[ignore]
#pattern = python-style regex for directories to ignore
pattern = ^\.|^bin$|^lib$|^apps$|^settings$|^eclipse$
I want it to search ~/data and two directories below it and to ignore all dotfiles and some other specific folders. (Note: No, it's not smart enough to not search the same folders if you give it nested folders like ~/,~/code, so don't do that.)
If you change the config file, you need to restart the daemon to reload it:
killall unity-filesearch-daemon
I packaged it so you can download it from my ppa.
sudo add-apt-repository ppa:pydave/unity-lenses
sudo apt-get update
sudo apt-get install unity-place-filesearch
Then you need to reset unity before the lens will be noticed:
setsid unity
If you give it a path, it usually shows the path as one of the results (so you can open folders like you could in gnome-do).
One of the features I'm still trying to get working properly is the ability to type a path, a space, and then a search word to search in the path. Most of the time it works, but sometimes it fails to find anything (probably because the search is taking forever).
I do not think you can. These seems to be stored in memory only and lost every time the service is restarted.
You could however uninstall scopes that you do not want to use. You could also disable "remote" scopes by going to system settings, privacy, and turning off "Include Online Search Results".
Failing that, you could always remove the .service files from /usr/share/dbus-1/services/
I STRONGLY recommend AGAINST doing that, but it is an option.
You can also file a feature request with the lense/scope maintainer.
I believe the ability to save previous seach results and filters would be done at the lense/scope level and not at a unity level. Though they may not be true.
TL;DR; You can't as of today.
Best Answer
Unity makes use of Zeitgeist in its dash where it provides the user with easy access to its most and recently used data (files/folders/applications) as well as searching over the Zeitgeist FTS (Full Text Search) extension.
There is more development going on to bring the same features to a contacts dash where the user can browse his contacts by most/recently used.
From the Zeitgeist website:
So it uses metadata.
The 2nd part of your question is answered from another page from that site: Which technologies does Zeitgeist use?