On Tue, Sep 22, 2015 at 10:44 PM, Alvaro Herrera <alvherre@xxxxxxxxxxxxxxx> 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
