Re: ALTER TABLE ADD COLUMN fast default

On 02/20/2018 09:36 PM, Andres Freund wrote:
> Hi,
> 
> On 2018-02-20 21:28:40 +0100, Tomas Vondra wrote:
>> I don't quite understand why would this case need the TPC-H tests, or
>> why would TPC-H give us more than the very focused tests we've already
>> done.
> 
> Because a more complex query shows the cost of changing cache access
> costs better than a trivial query. Simplistic queries will often
> e.g. not show cost of additional branch predictor usage, because the
> branch history is large enough to fit the simple query. But once you go
> to a more complex query, and that's not necessarily the case anymore.
> 
> 
>> The first test was testing a fairly short query where any such
>> additional overhead would be much more obvious, compared to the TPC-H
>> queries that usually do a lot of other expensive stuff.
> 
> Unfortunately such reasoning IME doesn't work well with cpu-bound stuff.
> 

OK, point taken. I'll do the tests and report the results.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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