Don't print extra parens around expressions in extended stats

The code printing expressions for extended statistics doubled the
parens, producing results like ((a+1)), which is unnecessary and not
consistent with how we print expressions elsewhere.

Fixed by tweaking the code to produce just a single set of parens.

Reported by Mark Dilger, fix by me. Backpatch to 14, where support for
extended statistics on expressions was added.

Reported-by: Mark Dilger
Discussion: https://postgr.es/m/20210122040101.GF27167%40telsasoft.com

src/backend/utils/adt/ruleutils.c

@@ -1712,7 +1712,7 @@ pg_get_statisticsobj_worker(Oid statextid, bool columns_only, bool missing_ok)
 	{
 		Node	   *expr = (Node *) lfirst(lc);
 		char	   *str;
-		int			prettyFlags = PRETTYFLAG_INDENT;
+		int			prettyFlags = PRETTYFLAG_PAREN;
 
 		str = deparse_expression_pretty(expr, context, false, false,
 										prettyFlags, 0);

src/bin/pg_dump/t/002_pg_dump.pl

@@ -2756,7 +2756,7 @@ my %tests = (
 		create_sql   => 'CREATE STATISTICS dump_test.test_ext_stats_expr
 							ON (2 * col1) FROM dump_test.test_fifth_table',
 		regexp => qr/^
-			\QCREATE STATISTICS dump_test.test_ext_stats_expr ON ((2 * col1)) FROM dump_test.test_fifth_table;\E
+			\QCREATE STATISTICS dump_test.test_ext_stats_expr ON (2 * col1) FROM dump_test.test_fifth_table;\E
 		    /xms,
 		like =>
 		  { %full_runs, %dump_test_schema_runs, section_post_data => 1, },

src/test/regress/expected/create_table_like.out

@@ -417,7 +417,7 @@ Check constraints:
     "ctlt1_a_check" CHECK (length(a) > 2)
 Statistics objects:
     "public.ctlt_all_a_b_stat" ON a, b FROM ctlt_all
-    "public.ctlt_all_expr_stat" ON ((a || b)) FROM ctlt_all
+    "public.ctlt_all_expr_stat" ON (a || b) FROM ctlt_all
 
 SELECT c.relname, objsubid, description FROM pg_description, pg_index i, pg_class c WHERE classoid = 'pg_class'::regclass AND objoid = i.indexrelid AND c.oid = i.indexrelid AND i.indrelid = 'ctlt_all'::regclass ORDER BY c.relname, objsubid;
     relname     | objsubid | description 
@@ -457,7 +457,7 @@ Check constraints:
     "ctlt1_a_check" CHECK (length(a) > 2)
 Statistics objects:
     "public.pg_attrdef_a_b_stat" ON a, b FROM public.pg_attrdef
-    "public.pg_attrdef_expr_stat" ON ((a || b)) FROM public.pg_attrdef
+    "public.pg_attrdef_expr_stat" ON (a || b) FROM public.pg_attrdef
 
 DROP TABLE public.pg_attrdef;
 -- Check that LIKE isn't confused when new table masks the old, either
@@ -479,7 +479,7 @@ Check constraints:
     "ctlt1_a_check" CHECK (length(a) > 2)
 Statistics objects:
     "ctl_schema.ctlt1_a_b_stat" ON a, b FROM ctlt1
-    "ctl_schema.ctlt1_expr_stat" ON ((a || b)) FROM ctlt1
+    "ctl_schema.ctlt1_expr_stat" ON (a || b) FROM ctlt1
 
 ROLLBACK;
 DROP TABLE ctlt1, ctlt2, ctlt3, ctlt4, ctlt12_storage, ctlt12_comments, ctlt1_inh, ctlt13_inh, ctlt13_like, ctlt_all, ctla, ctlb CASCADE;