[wp-hackers] Why URL in GUID?

[Owen Winkler]

Paul Menard wrote:
> The only I have is if this value must be unique, why is it not a unique 
> key in the database. More specific, why is it not 'the' unique key?

Because it's not the "ID" field, it's the "Globally Unique ID" field.

That means the value stored in that field must be unique not just for 
one site, but for any site anywhere.  The GUID is used, for one example, 
to differentiate posts on different sites for feed aggregators.

Presumably, if you switched sites and also changed the URL in your 
aggregator to the new setting, it would know not to re-load posts with 
the old GUIDs because they haven't changed.  Unfortunately, there is 
plenty of aggregator software that doesn't obey the spec, and when they 
ignore this, it causes some issues.  But you can also see why the GUID 
should not change when you change the slug.

You can't use the ID field of the posts table as the GUID because that 
value is *very* common among all blogs.  Creating hashes from existing 
data is never going to produce a more globally unique ID for a post than 
the actual URL for the post.

Owen