Re: Exposing the Xact commit order to the user

On 6/2/2010 7:49 PM, Greg Stark wrote:
> On Wed, Jun 2, 2010 at 6:45 PM, Chris Browne <cbbrowne(at)acm(dot)org> wrote:
>> It would make it easy to conclude:
>>
>> "This next transaction did 8328194 updates. Maybe we should do
>> some kind of checkpoint (e.g. - commit transaction or such) before
>> working on it."
>>
>> versus
>>
>> "This transaction we're thinking of working on had 7 updates. No
>> big deal..."
>
> I'm puzzled how you would define this value. How do you add 7 inserts,
> 7 deletes, and 7 updates? Is that 21 rows modified? Why are the 7
> inserts and 7 deletes worth twice as much as the 7 updates when
> they're basically the same thing? What if the inserts fired triggers
> which inserted 7 more rows, is that 14? What if the 7 updates modified
> 2 TB of TOAST data but the 8238194 updates were all to the same record
> and they were all HOT updates so all it did was change 8kB?
>
> In any case you'll have all the actual data from your triggers or
> hooks or whatever so what value does having the system keep track of
> this add?

The point is not that we don't have that information now. The point is
having a hint BEFORE wading through possibly gigabytes of WAL or log data.

If getting that information requires to read all the log data twice or
the need to read gigabytes of otherwise useless WAL data (as per Bruce's
suggestion), we better not get it at all and just keep doing what we are
doing now.

I actually have a hard time understanding why people are so opposed to a
feature that has zero impact at all unless a DBA actually turns in ON.
What is the problem with exposing the commit order of transactions?

Jan

--
Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin