This isn't about where to put the id of a piece of unique content in URLs, but more about densely packing the URL (or, does it just not matter).
Take for example, a hypothetical post in a blog:
The ID that uniquely identifies this is 123456789. This corresponds to a look-up and is the direct key in the underlying data store.
However, I could encode that in say, hexadecimal, like so:
And that would be shorter.
One could take it even further and have more compact encodings; since URLs are case sensitive, one could imagine an encoding that uses numbers, lowercase and uppercase letters, for a base of 62 (26 upper case + 26 lower case + 10 digits):
For a resulting URL of:
The question is, does densely packing the ID of the content (the requirement is that an ID is mandatory for look-ups) have a negative impact on SEO (and dare I ask, might it have any positive impact), or is it just not worth the time?
Note that this is not for a URL shortening service, so saving space in the URL for browser limitation purposes is not an issue.