Avoid permission failure in pg_sequences.last_value

Before, reading pg_sequences.last_value would fail unless the user had
appropriate sequence permissions, which would make the pg_sequences view
cumbersome to use.  Instead, return null instead of the real value when
there are no permissions.

From: Michael Paquier <michael.paquier@gmail.com>
Reported-by: Shinoda, Noriyoshi <noriyoshi.shinoda@hpe.com>

doc/src/sgml/catalogs.sgml

@@ -9882,7 +9882,10 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
       <entry></entry>
       <entry>The last sequence value written to disk.  If caching is used,
        this value can be greater than the last value handed out from the
-       sequence.  Null if the sequence has not been read from yet.</entry>
+       sequence.  Null if the sequence has not been read from yet.  Also, if
+       the current user does not have <literal>USAGE</literal>
+       or <literal>SELECT</literal> privilege on the sequence, the value is
+       null.</entry>
      </row>
     </tbody>
    </tgroup>

src/backend/catalog/system_views.sql

@@ -175,7 +175,11 @@ CREATE OR REPLACE VIEW pg_sequences AS
         S.seqincrement AS increment_by,
         S.seqcycle AS cycle,
         S.seqcache AS cache_size,
-        pg_sequence_last_value(C.oid) AS last_value
+        CASE
+            WHEN has_sequence_privilege(C.oid, 'SELECT,USAGE'::text)
+                THEN pg_sequence_last_value(C.oid)
+            ELSE NULL
+        END AS last_value
     FROM pg_sequence S JOIN pg_class C ON (C.oid = S.seqrelid)
          LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
     WHERE NOT pg_is_other_temp_schema(N.oid)

src/test/regress/expected/rules.out

@@ -1647,7 +1647,10 @@ pg_sequences| SELECT n.nspname AS schemaname,
     s.seqincrement AS increment_by,
     s.seqcycle AS cycle,
     s.seqcache AS cache_size,
-    pg_sequence_last_value((c.oid)::regclass) AS last_value
+        CASE
+            WHEN has_sequence_privilege(c.oid, 'SELECT,USAGE'::text) THEN pg_sequence_last_value((c.oid)::regclass)
+            ELSE NULL::bigint
+        END AS last_value
    FROM ((pg_sequence s
      JOIN pg_class c ON ((c.oid = s.seqrelid)))
      LEFT JOIN pg_namespace n ON ((n.oid = c.relnamespace)))