The GRAPH engine allows you to deal with hierarchies and graphs in a purely relational way. So, we can find all children of an item, path from an item to a root node, shortest path between two items, and so on, each with a simple basic query structure using standard SQL grammar.

The engine is implemented as a MySQL/MariaDB 5.1 plugin (we’re working on a 5.0 backport for some clients) and thus runs with an unmodified server.

Demo time! I’ll simplify/strip a little bit here for space reasons, but what’s here is plain cut/paste from a running server, no edits

-- insert a few entries with connections (and multiple paths)
insert into foo (origid, destid) values (1,2), (2,3), (2,4), (4,5), (3,6), (5,6);
-- a regular table to join on to
insert into people values (1,"pearce"),(2,"hunnicut"),(3,"potter"), …

Jeff Rothschild of Facebook’s “A Match Made in Heaven? The Social Graph and the Database”

Taking a look at the social graph and what it means for the database.

The social graph:

• At it’s heart it’s about people and their connections.
  
• Learning about people who are in your world.
  
• Can be a powerful tool for accelerating the use of an application.
  

“The social graph has transformed a seemingly simple application such as photos into something tremendously more powerful.” We’re interested about what people are saying about us, and about our friends. Social applications are compelling.

Facebook users blew through the estimate for 6 months of storage in 6 weeks. It is serving 250,000 photos per second at peak time, not including profiles. Facebook serves more photos than even the photo sites out there, and serves more event invitations …

In response to brad's post and daveman692's post, about Thoughts on the Social Graph.

I have posted the following:

I came home from FOOcamp with my mind buzzing with something similar. It was in the zeitgeist, I guess.

Between being annoyed at some of 6As recent actions, and annoyed that if I left LJ, I would lose a lot of valuable social network information. And then the sessions on distributed social networks, openid, …