Re: [HACKERS] Big IN() clauses etc : feature proposal

On Thu, May 11, 2006 at 12:18:06PM -0500, Jim C. Nasby wrote:
> > Yes, because there can be more than one active snapshot within a single
> > transaction (think about volatile functions in particular).
>
> Any documentation on how snapshot's work? They're a big mystery to me.
> :(

A snapshot is a particular view on a database. In particular, you have
to be able to view a version of the database that doesn't have you own
changes, otherwise an UPDATE would keep updating the same tuple. Also,
for example, a cursor might see an older version of the database than
queries being run. I don't know of any particular information about it
though. Google wasn't that helpful.

> > No; you forgot about subtransactions.
>
> Oh, I thought those were done with cmin and cmax... if that's not what
> cmin/cmax are for, then what is?

cmin/cmax are command counters. So in the sequence:

BEGIN;
SELECT 1;
SELECT 2;

The second query runs as the same transaction ID but a higher command
ID so it can see the result of the previous query. Subtransactions are
(AIUI anyway) done by having transactions depend on other transactions.
When you start a savepoint you start a new transaction ID whose status
is tied to its top-level transaction ID but can also be individually
rolledback.

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.