Re: On disable_cost

Robert Haas <robertmhaas@gmail.com>

From: Robert Haas <robertmhaas@gmail.com>

To: Peter Geoghegan <pg@bowt.ie>

Cc: David Rowley <dgrowleyml@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2024-06-12T15:35:48Z

Lists: pgsql-hackers

Attachments

v2-0001-Remove-grotty-use-of-disable_cost-for-TID-scan-pl.patch (application/octet-stream) patch v2-0001
v2-0002-Rationalize-behavior-of-enable_indexscan-and-enab.patch (application/octet-stream) patch v2-0002
v2-0003-Treat-the-of-disabled-nodes-in-a-path-as-a-separa.patch (application/octet-stream) patch v2-0003
v2-0004-Show-the-of-disabled-nodes-in-EXPLAIN-ANALYZE-out.patch (application/octet-stream) patch v2-0004

On Tue, May 7, 2024 at 4:19 PM Robert Haas <robertmhaas@gmail.com> wrote:
> Here are some patches for discussion.

Well, that didn't generate much discussion, but here I am trying
again. Here I've got patches 0001 and 0002 from my previous posting;
I've dropped 0003 and 0004 from the previous set for now so as not to
distract from the main event, but they may still be a good idea.
Instead I've got an 0003 and an 0004 that implement the "count of
disabled nodes" approach that we have discussed previously. This seems
to work fine, unlike the approaches I tried earlier. I think this is
the right direction to go, but I'd like to know what concerns people
might have.

This doesn't completely remove disable_cost, because hash joins still
add it to the cost when it's impossible to fit the MCV value into
work_mem. I'm not sure what to do with that. Continuing to use
disable_cost in that one scenario seems OK to me. We could
alternatively make that scenario bump disabled_nodes, but I don't
really want to confuse the planner not wanting to do something with
the user telling the planner not to do something, so I don't think
that's a good idea. Or we could rejigger things so that in that case
we don't generate the plan at all. I'm not sure why we don't do that
already, actually.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

Commits

Doc: add detail about EXPLAIN's "Disabled" property
- 84b8fccbe5c2 18.0 landed
Adjust EXPLAIN's output for disabled nodes
- 161320b4b960 18.0 landed
Fix order of parameters in a cost_sort call
- 87b6c3c0b703 18.0 landed
Show number of disabled nodes in EXPLAIN ANALYZE output.
- c01743aa4866 18.0 landed
Treat number of disabled nodes in a path as a separate cost metric.
- e22253467942 18.0 landed
Remove grotty use of disable_cost for TID scan plans.
- e4326fbc60c4 18.0 landed