Either I'm confused about the syntax description rules or the documentation for this aspect of window definitions is wrong.
The expression:
[ RANGE | ROWS ] BETWEEN frame_start AND frame_end
means that the specification of range or rows is optional - but that doesn't appear to be the case...
The documentation should read:
{ RANGE | ROWS } BETWEEN frame_start AND frame_end
Same goes for the bare frame_start specification preceding the between version.
The bare version looks funny without tag but "ORDER BY idx BETWEEN 5 PRECEDING AND CURRENT ROW" isn't obviously wrong until you get a syntax error...
David J.