Message-ID: <9e4ad616bebb103ec2084bf6f724cfc739e7fabb.camel@cybertec.at>
Subject: Re: On disable_cost
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>
Cc: David Rowley <dgrowleyml@gmail.com>, Heikki Linnakangas
 <hlinnaka@iki.fi>,  Peter Geoghegan <pg@bowt.ie>, PostgreSQL Hackers
 <pgsql-hackers@lists.postgresql.org>
Date: Fri, 27 Sep 2024 10:42:32 +0200
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On Wed, 2024-08-21 at 10:29 -0400, Robert Haas wrote:
> I went ahead and committed these patches. I know there's some debate
> over whether we want to show the # of disabled nodes and if so whether
> it should be controlled by COSTS, and I suspect I haven't completely
> allayed David's concerns about the initial_cost_XXX functions although
> I think that I did the right thing. But, I don't have the impression
> that anyone is desperately opposed to the basic concept, so I think it
> makes sense to put these into the tree and see what happens. We have
> quite a bit of time left in this release cycle to uncover bugs, hear
> from users or other developers, etc. about what problems there may be
> with this. If we end up deciding to reverse course or need to fix a
> bunch of stuff, so be it, but let's see what the feedback is.

I am somewhat unhappy about the "Disabled Nodes" in EXPLAIN.

First, the commit message confused me: it claims that the information
is displayed with EXPLAIN ANALYZE, but it's shown with every EXPLAIN.

But that's not important.  My complaints are:

1. The "disabled nodes" are always displayed.
   I'd be happier if it were only shown for COSTS ON, but I think it
   would be best if they were only shown with VERBOSE ON.

   After all, the messages are pretty verbose...

2. The "disabled nodes" are not only shown at the nodes where nodes
   were actually disabled, but also at every nodes above these nodes.

   This would be fine:

     Sort
       -> Nested Loop Join
            -> Hash Join
                 -> Index Scan
                    Disabled Nodes: 1
                 -> Hash
                      -> Index Scan
                         Disabled Nodes: 1
            -> Index Scan
               Disabled Nodes: 1

   This is annoying:

     Sort
     Disabled Nodes: 3
       -> Nested Loop Join
          Disabled Nodes: 3
            -> Hash Join
               Disabled Nodes: 2
                 -> Index Scan
                    Disabled Nodes: 1
                 -> Hash
                      -> Index Scan
                         Disabled Nodes: 1
            -> Index Scan
               Disabled Nodes: 1

I have no idea how #2 could be implemented, but it would be nice to have.
Please, please, can we show the "disabled nodes" only with VERBOSE?

Yours,
Laurenz Albe