On Tue, Sep 22, 2015 at 10:44 PM, Alvaro Herrera <alvhe...@2ndquadrant.com> wrote:
> Jeff Dik wrote: > > > 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. Thanks again! Jeff > > -- > Álvaro Herrera http://www.2ndQuadrant.com/ > PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services >
