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

On Thu, May 11, 2006 at 08:03:19PM +0200, Martijn van Oosterhout wrote:
> 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.

Ahh, I'd forgotten that commands sometimes needed to see prior data. But
that's done with cmin/max, right?

In any case, going back to the original thought/question... my point was
that in a single-session table, it should be possible to maintain a
row counter. Worst case, you might have to keep a seperate count for
each CID or XID, but that doesn't seem that unreasonable for a single
backend to do, unless you end up running a heck of a lot of commands.
More importantnly, it seems a lot more feasable to at least know how
many rows there are every time you COMMIT, which means you can
potentially avoid having to ANALYZE.

> > > 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.

Hmmm, interesting. I would have thought it was tied to CID, but I guess
XID has more of that machinery around to support rollback.
--
Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461