Re: On disable_cost

On Wed, 2024-10-02 at 21:55 +1300, David Rowley wrote:
> On Tue, 1 Oct 2024 at 06:17, Laurenz Albe <laurenz.albe@cybertec.at> wrote:
> > Why did you change "Disabled" from an integer to a boolean?
> 
> I just don't think "Disabled Nodes" is all that self-documenting and
> I'm also unsure why the full integer value of disabled_nodes is
> required over just displaying the boolean value of if the node is
> disabled or not. Won't readers look at the remainder of the plan to
> determine information about which other nodes are disabled? Do we need
> to give them a running total?

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?

Yours,
Laurenz Albe