Add test for pg_stat_reset_single_table_counters() on index

stats.sql is already doing some tests coverage on index statistics, by
retrieving for example idx_scan and friends in pg_stat_all_tables.
pg_stat_reset_single_table_counters() is supported for an index for a
long time, but the case was never covered.

This commit closes the gap, by using this reset function on an index,
cross-checking the contents of pg_stat_all_indexes.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/aN8l182jKxEq1h9f@paquier.xyz

src/test/regress/expected/stats.out

@@ -850,6 +850,27 @@ FROM pg_stat_all_tables WHERE relid = 'test_last_scan'::regclass;
         2 | t      |        3 | t
 (1 row)
 
+-- check the stats in pg_stat_all_indexes
+SELECT idx_scan, :'test_last_idx' < last_idx_scan AS idx_ok
+  FROM pg_stat_all_indexes WHERE indexrelid = 'test_last_scan_pkey'::regclass;
+ idx_scan | idx_ok 
+----------+--------
+        3 | t
+(1 row)
+
+-- check that the stats in pg_stat_all_indexes are reset
+SELECT pg_stat_reset_single_table_counters('test_last_scan_pkey'::regclass);
+ pg_stat_reset_single_table_counters 
+-------------------------------------
+ 
+(1 row)
+
+SELECT idx_scan FROM pg_stat_all_indexes WHERE indexrelid = 'test_last_scan_pkey'::regclass;
+ idx_scan 
+----------
+        0
+(1 row)
+
 -----
 -- Test reset of some stats for shared table
 -----

src/test/regress/sql/stats.sql

@@ -382,6 +382,15 @@ 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;
 
+-- check the stats in pg_stat_all_indexes
+SELECT idx_scan, :'test_last_idx' < last_idx_scan AS idx_ok
+  FROM pg_stat_all_indexes WHERE indexrelid = 'test_last_scan_pkey'::regclass;
+
+-- check that the stats in pg_stat_all_indexes are reset
+SELECT pg_stat_reset_single_table_counters('test_last_scan_pkey'::regclass);
+
+SELECT idx_scan FROM pg_stat_all_indexes WHERE indexrelid = 'test_last_scan_pkey'::regclass;
+
 -----
 -- Test reset of some stats for shared table
 -----