> I'd really love to learn: > > 1. Why the xmax for foo_id1 goes from 696 to 1 and what does that > mean?
When two transactions want to lock the same row, the xmax field is a multixact, no longer a bare transaction ID. This is an object that resolves to multiple transaction IDs. > 2. How does transaction A know it needs to take a ShareLock on > transaction B?
Because it reads the two transaction ID values from pg_multixact. > 3. What is a virtualtransaction and what do its numerator and denominator > mean?
It's not a division operation (so no numerator/denominator). The part before the / is a backend ID and the part after the / is a local transaction counter. It's just an identifier for the transaction, useful for the time before the transaction acquires a transaction ID. This optimizes that a transaction that doesn't modify tuples does not need to acquire a transaction ID (and thus keeps transaction ID consumption rate low.)
Thanks so much for these explanations!
Is there any way to inspect a multixact via psql to see what transaction ID values it has? I wasn't able to find anything while searching for an hour or so.