Revert "Optimize order of GROUP BY keys".

This reverts commit db0d67d and
several follow-on fixes.  The idea of making a cost-based choice
of the order of the sorting columns is not fundamentally unsound,
but it requires cost information and data statistics that we don't
really have.  For example, relying on procost to distinguish the
relative costs of different sort comparators is pretty pointless
so long as most such comparator functions are labeled with cost 1.0.
Moreover, estimating the number of comparisons done by Quicksort
requires more than just an estimate of the number of distinct values
in the input: you also need some idea of the sizes of the larger
groups, if you want an estimate that's good to better than a factor of
three or so.  That's data that's often unknown or not very reliable.
Worse, to arrive at estimates of the number of calls made to the
lower-order-column comparison functions, the code needs to make
estimates of the numbers of distinct values of multiple columns,
which are necessarily even less trustworthy than per-column stats.
Even if all the inputs are perfectly reliable, the cost algorithm
as-implemented cannot offer useful information about how to order
sorting columns beyond the point at which the average group size
is estimated to drop to 1.

Close inspection of the code added by db0d67d shows that there
are also multiple small bugs.  These could have been fixed, but
there's not much point if we don't trust the estimates to be
accurate in-principle.

Finally, the changes in cost_sort's behavior made for very large
changes (often a factor of 2 or so) in the cost estimates for all
sorting operations, not only those for multi-column GROUP BY.
That naturally changes plan choices in many situations, and there's
precious little evidence to show that the changes are for the better.
Given the above doubts about whether the new estimates are really
trustworthy, it's hard to summon much confidence that these changes
are better on the average.

Since we're hard up against the release deadline for v15, let's
revert these changes for now.  We can always try again later.

Note: in v15, I left T_PathKeyInfo in place in nodes.h even though
it's unreferenced.  Removing it would be an ABI break, and it seems
a bit late in the release cycle for that.

Discussion: https://postgr.es/m/TYAPR01MB586665EB5FB2C3807E893941F5579@TYAPR01MB5866.jpnprd01.prod.outlook.com

Author: tglsfdc

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -2840,13 +2840,16 @@ select c2 * (random() <= 1)::int as c2 from ft2 group by c2 * (random() <= 1)::i
 -- GROUP BY clause in various forms, cardinal, alias and constant expression
 explain (verbose, costs off)
 select count(c2) w, c2 x, 5 y, 7.0 z from ft1 group by 2, y, 9.0::int order by 2;
-                                                 QUERY PLAN                                                 
-------------------------------------------------------------------------------------------------------------
- Foreign Scan
+                                      QUERY PLAN                                       
+---------------------------------------------------------------------------------------
+ Sort
    Output: (count(c2)), c2, 5, 7.0, 9
-   Relations: Aggregate on (public.ft1)
-   Remote SQL: SELECT count(c2), c2, 5, 7.0, 9 FROM "S 1"."T 1" GROUP BY 2, 3, 5 ORDER BY c2 ASC NULLS LAST
-(4 rows)
+   Sort Key: ft1.c2
+   ->  Foreign Scan
+         Output: (count(c2)), c2, 5, 7.0, 9
+         Relations: Aggregate on (public.ft1)
+         Remote SQL: SELECT count(c2), c2, 5, 7.0, 9 FROM "S 1"."T 1" GROUP BY 2, 3, 5
+(7 rows)
 
 select count(c2) w, c2 x, 5 y, 7.0 z from ft1 group by 2, y, 9.0::int order by 2;
   w  | x | y |  z  

doc/src/sgml/config.sgml

@@ -5055,20 +5055,6 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-enable-groupby-reordering" xreflabel="enable_group_by_reordering">
-      <term><varname>enable_group_by_reordering</varname> (<type>boolean</type>)
-      <indexterm>
-       <primary><varname>enable_group_by_reordering</varname> configuration parameter</primary>
-      </indexterm>
-      </term>
-      <listitem>
-       <para>
-        Enables or disables reordering of keys in a <literal>GROUP BY</literal>
-        clause. The default is <literal>on</literal>.
-       </para>
-      </listitem>
-     </varlistentry>
-
      <varlistentry id="guc-enable-hashagg" xreflabel="enable_hashagg">
       <term><varname>enable_hashagg</varname> (<type>boolean</type>)
       <indexterm>