I would make the autocompletion dependent to the number of potential results. The dropdown must not contain of hunderts of items so you get it only when the target might be easy to find. In the example it's good with the first letter, but in case of more than 20 options (to name an arbitrary number) it's better to expand only when two or more charcters are entered (and to have an explicite way to show the list of options by pressing F2, for example), and finally for the AND/OR/NOT combinations you could expand immediately.