It's unimportant in the extreme to know all the various molecules you might find in any given herb.
It's rather good to know if your herb contains one or the other large group of constituents, though.
It's tied in with the extraction in various solvents, and it's tied in with how a herb might work, if you use it in material doses (= larger amounts, as in teas, or lotsa dropsa tincture).
But to go into the most minute details, constituent-wise? I wouldn't.
Try this: look up yarrow in the duke database:
1) clicky here.
2) type in yarrow and clicky on the "submit query" button.
3) mark the [ ] "Search based on non-ubiquitous chemicals only" box (... or don't mark it ...) and clicky on the "submit query" button.
4) now infer, from that list, how yarrow will work as a herb.
Can do it? Congratulations, I'll certify you as a very powerful herbal psychic on the spot!
Can't do it? I'm not surprised: nobody else can, either.
No, instead of looking at things from the details on up, it's better to look at them from large groups on down. It's much easier to tell what yarrow could be used for from its major constituent groups.
Of course, the pharmacognosists (that's the guys'n'gals who peer at single constituents, and do nothing but, all day long) are as bad as the botanists, and you'll find different lists of major constituents in all the various pharmacognosy textbooks. And these lists don't overlap ...
... here, then, are the constituent groups which I think of as important. They'll appear over the next few days as single entry blog posts.
Other entries: Absorption