deltas in web site releases

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 a web page can be treated as a directed graph, so a web site can be
understood as a collection of directed graphs
 I know there are CMS that keep whole web sites in databases, but I was
wondering how this is done. For example, given a number of paragraphs with
the same prior path on a page how do they distinguish the 3rd from the 7th
if they use just the path to the particular node.
 Checking the Last-Modified HTTP header is way to coarse. Isn't it?
 What about if you want to only specify exactly what has been modified and
where (in the page graphs), so that the page can be minimally updated.
 What is the smart ways to do something like that?
 I am a total newbie on the subject, but I have always wondered why things
are done that way (such things as "Last-Modified"). Any thoughtful
discussion on the subject?

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