So.... I talked about this patch with Ronan Dunklau and he had a good question.... Why are we maintaining relfrozenxid and relminmxid in pg_class for temporary tables at all? Autovacuum can't use them and other sessions won't care about them. The only session that might care about them is the one attached to the temp schema.
So we could replace relfrozenxid and relminmxid for temp tables with a local hash table that can be updated on every truncate easily and efficiently. If a temp table actually wraps around the only session that runs into problems is the one attached to that temp schema. It can throw local session errors and doesn't need to interfere with the rest of the cluster in any way. It could even start running vacuums though I'm not convinced that's a great solution. At least I think so. I'm pretty sure about relfrozenxid but as always I don't really know how relminmxid works. I think we only need to worry about multixacts for subtransactions, all of which are in the same transaction -- does that even work that way? But this is really attractive since it means no catalog updates just for temp tables on every transaction and no wraparound cluster problems even if you have on-commit-preserve-rows tables. It really shouldn't be possible for a regular user to cause the whole cluster to run into problems just by creating a temp table and keeping a connection around a while.
