Re: pgsql: Add parallel-aware hash joins.

Hi,

On 2018-01-24 13:11:22 -0500, Robert Haas wrote:
> So for me, the additional hash index tests don't cost anything
> measurable and the additional hash join tests cost about a second. I
> think this probably accounts for why committers other than you keep
> "adding so much time to the regression tests". On modern hardware,
> the costs just don't matter.

I very much agree with the general sentiment, but a second of a 25s test
certainly isn't nothing. As I've just written a few messages upthread,
I think we can hide the overall timing costs to a much larger degree
than we're doing, but I don't think we need not pay attention at all.

> Now, how much should we care about the performance of software with a
> planned release date of 2018 on hardware discontinued in 2001,
> hardware that is apparently about 20 times slower than a modern
> laptop? Some, perhaps, but maybe not a whole lot. Removing tests
> that have found actual bugs because they cost runtime on ancient
> systems that nobody uses for serious work doesn't make sense to me.

I again agree with the sentiment. One caveat is that old machines also
somewhat approximate testing with more instrumentation / debugging
enabled (say valgrind, CLOBBER_CACHE_ALWAYS, etc). So removing excessive
test overhead has still quite some benefits. But I definitely do not
want to lower coverage to achieve it.

Greetings,

Andres Freund