[dmm]

In postgresql at least, serializable cannot deadlock. It uses predicate locks to provide serializable consistency with high levels of concurrency. You can have serialization failures though.

[basetop]

I'm not too familiar with postgresql, but isn't that serializable snapshot rather than serializable? I'm pretty sure all RDMBs have deadlocks in serializable. But in serialiable snapshot, a transaction doesn't deadlock, but simply fails.

[freels]

Serializable Snapshot an implementation strategy for providing Serializable isolation. In PostgreSQL if you ask for Serializable you get SSI.

[convolvatron]

a transaction system in which its impossible for transactions to ever fail kind of isn't a transaction system.

see above comment from abadid - its entirely possible to impose a global ordering of all transactions either up front or retroactively without admitting deadlocks.