v2?

dist_key.sql

@@ -1,4 +1,5 @@
 -- CREATE VIEW citus_shard_key_candidates AS
+WITH initial_suggestions AS (
     WITH fkeys AS (
         select distinct on (freloid, confkey)
             c.oid as reloid, cf.oid as freloid,
@@ -28,7 +29,7 @@ WITH fkeys AS (
             AND c.relkind = 'r' and c.relpersistence = 'p'
             AND cf.relkind = 'r' and cf.relpersistence = 'p'
     )
-select n.nspname as schema_name, c.relname as table_name,
+    select c.oid as table_oid,
            CASE WHEN pkeys.attname IS NULL THEN 'reference' ELSE 'distributed' END AS citus_table_type,
            pkeys.attname as distribution_key,
            CASE
@@ -67,6 +68,13 @@ select n.nspname as schema_name, c.relname as table_name,
               and d.deptype = 'e'
          )
        and not exists (select 1 from pg_dist_partition where logicalrelid = c.oid)
-group by 1,2,3,4
-order by citus_table_type, n.nspname, c.relname;
-
+    group by 1,2,3
+    order by citus_table_type, c.oid
+)
+SELECT conrelid, confrelid
+FROM initial_suggestions t1
+JOIN pg_constraint ON (conrelid = t1.table_oid)
+WHERE confrelid != 0 AND
+      -- find illegal conditions
+      t1.citus_table_type = 'reference' AND
+      EXISTS (SELECT 1 FROM initial_suggestions t2 WHERE t2.citus_table_type = 'distributed' AND t2.table_oid = confrelid);