gp_toolkit.gp_skew_* should support replicated table correctly

gp_toolkit.gp_skew_* series views/functions are used to query how data
is skewed in database. The idea is using a query like:
"select gp_segment_id, count(*) cnt from foo group by gp_segment_id",
and compare the cnt by gp_segment_id.

For the replicated table, only one replica is picked to count the tuple
number by the planner, so the old calculate logic produced a confusing
result that a replicated table is skewed which is not expected:

gpadmin=# select * From gp_toolkit.gp_skew_idle_fractions;
 sifoid | sifnamespace | sifrelname |      siffraction
 --------+--------------+------------+------------------------
   16385 | public       | rpt        | 0.66666666666666666667

What's more, gp_segment_id is ambiguous for replicated table, so in
commit b120194a, we disallow user to access system columns include
gp_segment_id, so gp_toolkit.gp_skew_* views now report an error
now.

This commit correct the results of gp_toolkit.gp_skew_*
views/functions for the replicated table although the results are
pointless, however, this way should be more friendly for users.

src/backend/catalog/gp_toolkit.sql

@@ -577,6 +577,7 @@ DECLARE
     skewaot bool;
     skewsegid int;
     skewtablename record;
+    skewreplicated record;
 
 BEGIN
 
@@ -602,18 +603,35 @@ BEGIN
         SELECT * INTO skewtablename FROM gp_toolkit.__gp_fullname
         WHERE fnoid = $1;
 
-        OPEN skewcrs
-            FOR
-            EXECUTE
-                'SELECT ' || $1 || '::oid, segid, CASE WHEN gp_segment_id IS NULL THEN 0 ELSE cnt END ' ||
-                'FROM (SELECT generate_series(0, numsegments - 1) FROM gp_toolkit.__gp_number_of_segments) segs(segid) ' ||
-                'LEFT OUTER JOIN ' ||
-                    '(SELECT gp_segment_id, COUNT(*) AS cnt FROM ' ||
+        SELECT * INTO skewreplicated FROM gp_distribution_policy WHERE policytype = 'r' AND localoid = $1;
+
+        IF FOUND THEN
+            -- replicated table, gp_segment_id is user-invisible and all replicas have same count of tuples.
+            OPEN skewcrs
+                FOR
+                EXECUTE
+                    'SELECT ' || $1 || '::oid, segid, ' ||
+                    '(' ||
+                        'SELECT COUNT(*) AS cnt FROM ' ||
                         quote_ident(skewtablename.fnnspname) ||
                         '.' ||
                         quote_ident(skewtablename.fnrelname) ||
-                    ' GROUP BY 1) details ' ||
-                'ON segid = gp_segment_id';
+                    ') '
+                    'FROM (SELECT generate_series(0, numsegments - 1) FROM gp_toolkit.__gp_number_of_segments) segs(segid)';
+        ELSE
+            OPEN skewcrs
+                FOR
+                EXECUTE
+                    'SELECT ' || $1 || '::oid, segid, CASE WHEN gp_segment_id IS NULL THEN 0 ELSE cnt END ' ||
+                    'FROM (SELECT generate_series(0, numsegments - 1) FROM gp_toolkit.__gp_number_of_segments) segs(segid) ' ||
+                    'LEFT OUTER JOIN ' ||
+                        '(SELECT gp_segment_id, COUNT(*) AS cnt FROM ' ||
+                            quote_ident(skewtablename.fnnspname) ||
+                            '.' ||
+                            quote_ident(skewtablename.fnrelname) ||
+                        ' GROUP BY 1) details ' ||
+                    'ON segid = gp_segment_id';
+        END IF;
 
         FOR skewsegid IN
             SELECT generate_series(1, numsegments)

src/include/catalog/catversion.h

@@ -56,6 +56,6 @@
  */
 
 /*							3yyymmddN */
-#define CATALOG_VERSION_NO	301901101
+#define CATALOG_VERSION_NO	301901111
 
 #endif

src/test/regress/expected/gp_toolkit.out

@@ -271,6 +271,29 @@ select * from gp_toolkit.gp_bloat_diag where bdirelid = 'toolkit_skew'::regclass
 ----------+------------+------------+-------------+-------------+---------
 (0 rows)
 
+-- Test that gp_toolkit.gp_skew* functions works for the replicated table.
+create table toolkit_skew_rpt (i int, j int) distributed replicated;
+insert into toolkit_skew_rpt select i, i from generate_series(1, 100) i;
+select segid, segtupcount FROM gp_toolkit.gp_skew_details('toolkit_skew_rpt'::regclass);
+ segid | segtupcount 
+-------+-------------
+     0 |         100
+     1 |         100
+     2 |         100
+(3 rows)
+
+select skccoeff from gp_toolkit.gp_skew_coefficient('toolkit_skew_rpt'::regclass);
+        skccoeff         
+-------------------------
+ 0.000000000000000000000
+(1 row)
+
+select siffraction from gp_toolkit.gp_skew_idle_fraction('toolkit_skew_rpt'::regclass);
+      siffraction       
+------------------------
+ 0.00000000000000000000
+(1 row)
+
 -- Make sure gp_toolkit.gp_bloat_expected_pages does not report partition roots
 create table do_not_report_partition_root (i int, j int) distributed by (i)
 partition by range(j)
@@ -629,13 +652,14 @@ drop resource queue q;
 -- view.
 select autnspname, autrelname, autrelkind, autoid::regclass, autrelacl
 from gp_toolkit.__gp_user_data_tables_readable where autrelname like 'toolkit%';
- autnspname |  autrelname  | autrelkind |       autoid        | autrelacl 
-------------+--------------+------------+---------------------+-----------
- public     | toolkit_ao   | r          | public.toolkit_ao   | 
- public     | toolkit_heap | r          | public.toolkit_heap | 
- public     | toolkit_skew | r          | public.toolkit_skew | 
- tktest     | toolkit_ao2  | r          | toolkit_ao2         | 
-(4 rows)
+ autnspname |    autrelname    | autrelkind |         autoid          | autrelacl 
+------------+------------------+------------+-------------------------+-----------
+ public     | toolkit_ao       | r          | public.toolkit_ao       | 
+ public     | toolkit_heap     | r          | public.toolkit_heap     | 
+ public     | toolkit_skew     | r          | public.toolkit_skew     | 
+ public     | toolkit_skew_rpt | r          | public.toolkit_skew_rpt | 
+ tktest     | toolkit_ao2      | r          | toolkit_ao2             | 
+(5 rows)
 
 -- Switch to non-privileged user, and test that they are no longer visible.
 set session authorization toolkit_user1;

src/test/regress/sql/gp_toolkit.sql

@@ -138,6 +138,13 @@ select btdrelpages > 0 as btdrelpages_over_0,
 from gp_toolkit.gp_bloat_expected_pages where btdrelid = 'toolkit_skew'::regclass;
 select * from gp_toolkit.gp_bloat_diag where bdirelid = 'toolkit_skew'::regclass;
 
+-- Test that gp_toolkit.gp_skew* functions works for the replicated table.
+create table toolkit_skew_rpt (i int, j int) distributed replicated;
+insert into toolkit_skew_rpt select i, i from generate_series(1, 100) i;
+select segid, segtupcount FROM gp_toolkit.gp_skew_details('toolkit_skew_rpt'::regclass);
+select skccoeff from gp_toolkit.gp_skew_coefficient('toolkit_skew_rpt'::regclass);
+select siffraction from gp_toolkit.gp_skew_idle_fraction('toolkit_skew_rpt'::regclass);
+
 -- Make sure gp_toolkit.gp_bloat_expected_pages does not report partition roots
 create table do_not_report_partition_root (i int, j int) distributed by (i)
 partition by range(j)