Re: PG15 beta1 sort performance regression due to Generation context change

On 7/15/22 6:40 PM, Jonathan S. Katz wrote:
> On 7/15/22 4:54 PM, Tom Lane wrote:
>> Tomas Vondra <tomas.vondra@enterprisedb.com> writes:
>>> ... My personal opinion is that it's a rare regression. Other
>>> optimization patches have similar rare regressions, except that David
>>> spent so much time investigating this one it seems more serious.
>>
>> Yeah, this.  I fear we're making a mountain out of a molehill.  We have
>> committed many optimizations that win on average but possibly lose
>> in edge cases, and not worried too much about it.
> 
> I disagree with the notion of this being a "mountain out of a molehill." 
> The RMT looked at the situation, asked if we should make one more pass. 
> There were logical argument as to why not to (e.g. v16 efforts). I think 
> that is reasonable, and we can move on from any additional code changes 
> for v15.
> 
> What I find interesting is the resistance to adding any documentation 
> around this feature to guide users in case they hit the regression. I 
> understand it can be difficult to provide guidance on issues related to 
> adjusting work_mem, but even just a hint in the release notes to say "if 
> you see a performance regression you may need to adjust work_mem" would 
> be helpful. This would help people who are planning upgrades to at least 
> know what to watch out for.
> 
> If that still seems unreasonable, I'll agree to disagree so we can move 
> on with other parts of the release.

For completeness, I marked the open item as closed.

Jonathan