What does unsigned in mysql mean?

What does it mean  when Facebook says it uses MySQL as a "bit store"?

• Aditya Agarwal said this in a presentation on Facebook Engineering a few years ago. I think it was on the Facebook website, but I can't find it now. What exactly did he mean by this?

• Answer:

  In the context of Agriwal's presentation, he means that they mostly use MySQL as a key/value store.  That is, they don't do complex SQL queries, they just look up individual rows by primary key and update them in the same manner MySQL's InnoDB storage engine is optimized for primary key access, so if you can pattern your queries to use individual rows and address them by primary key, you can achieve some great throughput. It takes some care to structure your data in this way, and it's not every application that can benefit from this strategy.  Developers get into trouble when they try to store data in a key/value format (one row per attribute), but still query the data as though it's stored in conventional relational structure.  When you really do need SQL operations to act on sets of rows (that is, tables), then you should store data in the traditional relational way.