Re: The curious case of two inserts, a shrinking xmax, and a ShareLock on transaction

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.)

--
Álvaro Herrera                http://www.2ndQuadrant.com/
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