On the left pane of Dolphin there're two nodes named "Recently Saved" and "Search For" and there're a lot of stuff, more than 15 GB altogether. Where did all those come from? Is it safe to delete all those?

Those are just quick links to different views which show things like files that have changed recently or files of certain types. You can right-click on the entries to Hide them, but you can't really delete them as they don't really exist (outside of Dolphin).
Also, the files they "contain" are files that dolphin (and/or the desktop indexing) found which match the different criteria, but which already exist in various parts of the file system. They are not duplicates and are not taking up extra space because they show up there (the 15 GB would just be the total size of items which matched the given criteria). Don't delete them unless you know that you really want to remove the file from its actual location.


If you are concerned with disk space you should use other tools such as Filelight to find where all the space is being taken up, or BleachBit to help determine what you can do safely to clean up your system. See also: https://www.omgubuntu.co.uk/2016/08/5-w ... -on-ubuntu for additional tips (there are also a few other similar blog posts which include different tools and things)