Re: Distinct performance dropped by multiple times in v16

On 6/10/24 13:59, Vitaliy Litovskiy wrote:
> 2.2 unnest is removed. it is not really needed for this particular data 
> but this query is autogenerated and unnest makes sense for other data
> 
> 2.3 "order by token" is uncommented, this is my current way of fixing 
> the problem I would really appreciate some feedback if that is expected 
> behaviour and if there are better solutions
After second thought I found that the key issue here is too cheap cycles 
other unnest() routine. unnest procost is 1 as any other routines but it 
looks a bit more costly than it is.
Also, you can tune cpu_operator_cost a bit. Right now it is set to 
0.0025 by default. Increasing it to 0.005:
SET cpu_operator_cost = 0.005;
resolves your issue.
I guess, the value of cpu_operator_cost usually not a problem, because 
table pages costs much more. But here function calls are the main source 
of load and because of that need to be estimated more precisely.
I hope this will be helpful for you.

-- 
regards,
Andrei Lepikhov
Postgres Professional