We are using MySQL at phabricator.wikimedia.org. Currently the search is set to OR by default (we will change that, see the discussion at https://phabricator.wikimedia.org/T89274). I expected the search engine to accept `AND` and `OR` as operators to change the default behaviour, but it doesn't accept that. I can't tell you how to "fix" that but to me it felt like a bug in the user interface that should be fixed in the base code that powers the search.