Horton

Re: On disable_cost

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Peter Geoghegan <pg@bowt.ie>, David Rowley <dgrowleyml@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-07-02T20:43:41Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> Well, I don't really know where to go from here. I mean, I think that
> three committers (David, Heikki, yourself) have expressed some
> concerns about changing the behavior. So maybe we shouldn't. But I
> don't understand how it's reasonable to have two very similarly named
> GUCs behave (1) inconsistently with each other and (2) in a way that
> cannot be guessed from the documentation.

If the documentation isn't adequate, that's certainly an improvable
situation.  It doesn't seem hard:

-        Enables or disables the query planner's use of index-scan plan
-        types. The default is <literal>on</literal>.
+        Enables or disables the query planner's use of index-scan plan
+        types (including index-only scans).
+        The default is <literal>on</literal>.

More to the point, if we do change the longstanding meaning of this
GUC, that will *also* require documentation work IMO.

			regards, tom lane

Commits

  1. Doc: add detail about EXPLAIN's "Disabled" property

  2. Adjust EXPLAIN's output for disabled nodes

  3. Fix order of parameters in a cost_sort call

  4. Show number of disabled nodes in EXPLAIN ANALYZE output.

  5. Treat number of disabled nodes in a path as a separate cost metric.

  6. Remove grotty use of disable_cost for TID scan plans.