Fix pg_stat_reset_single_table_counters() for shared relations

This commit fixes the function of $subject for shared relations.  This
feature has been added by e042678.  Unfortunately, this new behavior got
removed by 5891c7a when moving statistics to shared memory.

Reported-by: Mitsuru Hinata
Author: Masahiro Ikeda
Reviewed-by: Kyotaro Horiguchi, Masahiko Sawada
Discussion: https://postgr.es/m/7cc69f863d9b1bc677544e3accd0e4b4@oss.nttdata.com
Backpatch-through: 15

src/test/regress/sql/stats.sql

@@ -376,6 +376,36 @@ COMMIT;
 SELECT seq_scan, :'test_last_seq' = last_seq_scan AS seq_ok, idx_scan, :'test_last_idx' < last_idx_scan AS idx_ok
 FROM pg_stat_all_tables WHERE relid = 'test_last_scan'::regclass;
 
+-----
+-- Test reset of some stats for shared table
+-----
+
+-- This updates the comment of the database currently in use in
+-- pg_shdescription with a fake value, then sets it back to its
+-- original value.
+SELECT shobj_description(d.oid, 'pg_database') as description_before
+  FROM pg_database d WHERE datname = current_database() \gset
+
+-- force some stats in pg_shdescription.
+BEGIN;
+SELECT current_database() as datname \gset
+COMMENT ON DATABASE :"datname" IS 'This is a test comment';
+SELECT pg_stat_force_next_flush();
+COMMIT;
+
+-- check that the stats are reset.
+SELECT (n_tup_ins + n_tup_upd) > 0 AS has_data FROM pg_stat_all_tables
+  WHERE relid = 'pg_shdescription'::regclass;
+SELECT pg_stat_reset_single_table_counters('pg_shdescription'::regclass);
+SELECT (n_tup_ins + n_tup_upd) > 0 AS has_data FROM pg_stat_all_tables
+  WHERE relid = 'pg_shdescription'::regclass;
+
+-- set back comment
+\if :{?description_before}
+  COMMENT ON DATABASE :"datname" IS :'description_before';
+\else
+  COMMENT ON DATABASE :"datname" IS NULL;
+\endif
 
 -----
 -- Test that various stats views are being properly populated