Re: On disable_cost

On 03.10.2024 01:44, David Rowley wrote:
> On Thu, 3 Oct 2024 at 08:41, Alena Rybakina<a(dot)rybakina(at)postgrespro(dot)ru> wrote:
>> I may have misunderstood your message, but disabled nodes number must propagate up the tree, otherwise it will be incorrect.
> I think there are two misunderstandings on this thread:
>
> 1) You're misunderstanding what Laurenz is complaining about. He's
> only concerned with the EXPLAIN output, not how disasbled_nodes works
> internally.
Sorry, maybe you're right, I misunderstood his request [0]. But I tried
to answer his question why disabled nodes aren't displayed by explaining
how it works.
> 2) Laurenz is misunderstanding what "Disabled Nodes" means. It has
> nothing to do with other Paths which were considered and rejected. It
> might be better named as "Disabled Degree". It tracks how many plan
> nodes below and including this node are disabled.

yes, I agree with you and that's exactly what I tried to explain with
examples.

Unfortunately I was unable to generalize this conclusion correctly. Thanks)

> Because of #2, I think I now understand why Laurenz was interested in
> only showing this with VERBOSE. If it worked the way Laurenz thought,
> I'd probably agree with him.
>
> Overall, I think we need to do something here. There's no
> documentation about what Disabled Nodes means so we either need to
> make it easier to understand without documenting it or add something
> to the documents about it. If Laurenz, who has a huge amount of
> PostgreSQL experience didn't catch it, then what hope is there for the
> average user?

I think you are right, most users will perceive this parameter as the
number of rejected paths, and not in any other way.

[0]
https://www.postgresql.org/message-id/0cdd3504502aac827acb3ae615eda09aeb883f74.camel%40cybertec.at

--
Regards,
Alena Rybakina
Postgres Professional