On Thu, Sep 25, 2014 at 3:17 PM, Peter Geoghegan <pg@heroku.com> wrote:
>> To find out how much that optimization buys, you
>> should use tuples with many variable-length columns (say, 50)
>> preceding the text column you're sorting on. I won't be surprised if
>> that turns out to be expensive enough to be worth worrying about, but
>> I have not benchmarked it.
>
> Sorry, but I don't follow. I don't think the pertinent question is if
> it's a noticeable cost. I think the pertinent question is if it's
> worth it. Doing something about it necessitates a lot of extra memory
> access. Not doing something about it hardly affects the amount of
> memory access required, perhaps not at all.
I think you're mincing words. If you go back and fix datum1, you'll
spend a bunch of effort doing that. If you don't, you'll pay a cost
on every comparison to re-find the relevant column inside each tuple.
You can compare those costs in a variety of cases, including the one I
mentioned, where the latter cost will be relatively high.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company