Re: On disable_cost

Hi!

On 02.10.2024 21:04, Laurenz Albe wrote:
> I didn't want a running total, but maybe I misunderstood what a disabled
> node is; see below.
>
>>> If you see a join where two plans were disabled, that's useful information.
>> I'm not sure if I follow what you mean here.  The patch will show
>> "Disabled: true" for both the inner and outer side of the join if both
>> of those are disabled.  The difference is that my patch does not show
>> the join itself is disabled like master does. I thought that's what
>> you were complaining about. Can you show an example of what you mean?
> I ran the following example, and now I am confused.
>
> CREATE TABLE tab_a (id integer);
>
> CREATE TABLE tab_b (id integer);
>
> SET enable_nestloop = off;
> SET enable_hashjoin = off;
>
> EXPLAIN SELECT * FROM tab_a JOIN tab_b USING (id);
>
> QUERY PLAN
> ═════════════════════════════════════════════════════════════════════
> Merge Join (cost=359.57..860.00 rows=32512 width=4)
> Merge Cond: (tab_a.id = tab_b.id)
> -> Sort (cost=179.78..186.16 rows=2550 width=4)
> Sort Key: tab_a.id
> -> Seq Scan on tab_a (cost=0.00..35.50 rows=2550 width=4)
> -> Sort (cost=179.78..186.16 rows=2550 width=4)
> Sort Key: tab_b.id
> -> Seq Scan on tab_b (cost=0.00..35.50 rows=2550 width=4)
>
> I would have expected to see "Disabled nodes: 2" with the merge join,
> because both the nested loop join and the hash join have been disabled.
>
> Why is there no disabled node shown?
>
Disabled nodes show the number of disabled paths, you simply don’t have
them here in mergejoin, because hashjoin and nestedloop were not
selected. The reason is the compare_path_costs_fuzzily function, because
the function decides which path is better based on fewer disabled nodes.
hashjoin and nestedloop have 1 more nodes compared to mergejoin. you can
disable mergejoin, I think the output about this will appear.

--
Regards,
Alena Rybakina
Postgres Professional