Show sort ordering options in EXPLAIN output.

Up to now, EXPLAIN has contented itself with printing the sort expressions
in a Sort or Merge Append plan node.  This patch improves that by
annotating the sort keys with COLLATE, DESC, USING, and/or NULLS FIRST/LAST
whenever nondefault sort ordering options are used.  The output is now a
reasonably close approximation of an ORDER BY clause equivalent to the
plan's ordering.

Marius Timmer, Lukas Kreft, and Arne Scheffer; reviewed by Mike Blackwell.
Some additional hacking by me.

src/test/regress/expected/aggregates.out

@@ -735,7 +735,7 @@ explain (costs off)
                              QUERY PLAN                              
 ---------------------------------------------------------------------
  Sort
-   Sort Key: (generate_series(1, 3))
+   Sort Key: (generate_series(1, 3)) DESC
    InitPlan 1 (returns $0)
      ->  Limit
            ->  Index Only Scan Backward using tenk1_unique2 on tenk1
@@ -784,7 +784,7 @@ explain (costs off)
    InitPlan 2 (returns $1)
      ->  Limit
            ->  Merge Append
-                 Sort Key: minmaxtest_1.f1
+                 Sort Key: minmaxtest_1.f1 DESC
                  ->  Index Only Scan Backward using minmaxtesti on minmaxtest minmaxtest_1
                        Index Cond: (f1 IS NOT NULL)
                  ->  Index Only Scan Backward using minmaxtest1i on minmaxtest1 minmaxtest1_1
@@ -823,7 +823,7 @@ explain (costs off)
    InitPlan 2 (returns $1)
      ->  Limit
            ->  Merge Append
-                 Sort Key: minmaxtest_1.f1
+                 Sort Key: minmaxtest_1.f1 DESC
                  ->  Index Only Scan Backward using minmaxtesti on minmaxtest minmaxtest_1
                        Index Cond: (f1 IS NOT NULL)
                  ->  Index Only Scan Backward using minmaxtest1i on minmaxtest1 minmaxtest1_1

src/test/regress/expected/collate.out

@@ -603,6 +603,25 @@ ALTER TABLE collate_test22 ADD FOREIGN KEY (f2) REFERENCES collate_test20;
 RESET enable_seqscan;
 RESET enable_hashjoin;
 RESET enable_nestloop;
+-- EXPLAIN
+EXPLAIN (COSTS OFF)
+  SELECT * FROM collate_test10 ORDER BY x, y;
+                  QUERY PLAN                  
+----------------------------------------------
+ Sort
+   Sort Key: x COLLATE "C", y COLLATE "POSIX"
+   ->  Seq Scan on collate_test10
+(3 rows)
+
+EXPLAIN (COSTS OFF)
+  SELECT * FROM collate_test10 ORDER BY x DESC, y COLLATE "C" ASC NULLS FIRST;
+                        QUERY PLAN                         
+-----------------------------------------------------------
+ Sort
+   Sort Key: x COLLATE "C" DESC, y COLLATE "C" NULLS FIRST
+   ->  Seq Scan on collate_test10
+(3 rows)
+
 -- 9.1 bug with useless COLLATE in an expression subject to length coercion
 CREATE TEMP TABLE vctable (f1 varchar(25));
 INSERT INTO vctable VALUES ('foo' COLLATE "C");

src/test/regress/expected/equivclass.out

@@ -319,7 +319,7 @@ explain (costs off)
                      ->  Index Scan using ec1_expr4 on ec1 ec1_3
                ->  Materialize
                      ->  Sort
-                           Sort Key: ec1.f1
+                           Sort Key: ec1.f1 USING <
                            ->  Index Scan using ec1_pkey on ec1
                                  Index Cond: (ff = 42::bigint)
 (20 rows)
@@ -376,7 +376,7 @@ explain (costs off)
          ->  Index Scan using ec1_expr4 on ec1 ec1_3
    ->  Materialize
          ->  Sort
-               Sort Key: ec1.f1
+               Sort Key: ec1.f1 USING <
                ->  Index Scan using ec1_pkey on ec1
                      Index Cond: (ff = 42::bigint)
 (14 rows)

src/test/regress/sql/collate.sql

@@ -220,6 +220,15 @@ RESET enable_seqscan;
 RESET enable_hashjoin;
 RESET enable_nestloop;
 
+
+-- EXPLAIN
+
+EXPLAIN (COSTS OFF)
+  SELECT * FROM collate_test10 ORDER BY x, y;
+EXPLAIN (COSTS OFF)
+  SELECT * FROM collate_test10 ORDER BY x DESC, y COLLATE "C" ASC NULLS FIRST;
+
+
 -- 9.1 bug with useless COLLATE in an expression subject to length coercion
 
 CREATE TEMP TABLE vctable (f1 varchar(25));