Add sequence USAGE privileges to information schema

The sequence USAGE privilege is sufficiently similar to the SQL
standard that it seems reasonable to show in the information schema.
Also add some compatibility notes about it on the GRANT reference
page.

doc/src/sgml/information_schema.sgml

@@ -3839,7 +3839,7 @@ ORDER BY c.ordinal_position;
      <row>
       <entry><literal>object_type</literal></entry>
       <entry><type>character_data</type></entry>
-      <entry><literal>COLLATION</literal> or <literal>DOMAIN</literal> or <literal>FOREIGN DATA WRAPPER</literal> or <literal>FOREIGN SERVER</literal></entry>
+      <entry><literal>COLLATION</literal> or <literal>DOMAIN</literal> or <literal>FOREIGN DATA WRAPPER</literal> or <literal>FOREIGN SERVER</literal> or <literal>SEQUENCE</literal></entry>
      </row>
 
      <row>
@@ -5859,7 +5859,7 @@ ORDER BY c.ordinal_position;
    <literal>USAGE</literal> privileges granted on various kinds of
    objects to a currently enabled role or by a currently enabled role.
    In <productname>PostgreSQL</productname>, this currently applies to
-   collations, domains, foreign-data wrappers, and foreign servers.  There is one
+   collations, domains, foreign-data wrappers, foreign servers, and sequences.  There is one
    row for each combination of object, grantor, and grantee.
   </para>
 
@@ -5871,6 +5871,13 @@ ORDER BY c.ordinal_position;
    object types, however, show real privileges.
   </para>
 
+  <para>
+   In PostgreSQL, sequences also support <literal>SELECT</literal>
+   and <literal>UPDATE</literal> privileges in addition to
+   the <literal>USAGE</literal> privilege.  These are nonstandard and therefore
+   not visible in the information schema.
+  </para>
+
   <table>
    <title><literal>usage_privileges</literal> Columns</title>
 
@@ -5918,7 +5925,7 @@ ORDER BY c.ordinal_position;
      <row>
       <entry><literal>object_type</literal></entry>
       <entry><type>character_data</type></entry>
-      <entry><literal>COLLATION</literal> or <literal>DOMAIN</literal> or <literal>FOREIGN DATA WRAPPER</literal> or <literal>FOREIGN SERVER</literal></entry>
+      <entry><literal>COLLATION</literal> or <literal>DOMAIN</literal> or <literal>FOREIGN DATA WRAPPER</literal> or <literal>FOREIGN SERVER</literal> or <literal>SEQUENCE</literal></entry>
      </row>
 
      <row>

doc/src/sgml/ref/grant.sgml

@@ -642,6 +642,18 @@ GRANT admins TO joe;
     translations.
    </para>
 
+   <para>
+    In the SQL standard, sequences only have a <literal>USAGE</literal>
+    privilege, which controls the use of the <literal>NEXT VALUE FOR</literal>
+    expression, which is equivalent to the
+    function <function>nextval</function> in PostgreSQL.  The sequence
+    privileges <literal>SELECT</literal> and <literal>UPDATE</literal> are
+    PostgreSQL extensions.  The application of the
+    sequence <literal>USAGE</literal> privilege to
+    the <literal>currval</literal> function is also a PostgreSQL extension (as
+    is the function itself).
+   </para>
+
    <para>
     Privileges on databases, tablespaces, schemas, and languages are
     <productname>PostgreSQL</productname> extensions.

src/backend/catalog/information_schema.sql

@@ -2212,6 +2212,43 @@ CREATE VIEW usage_privileges AS
     WHERE u_grantor.oid = srv.grantor
           AND grantee.oid = srv.grantee
           AND srv.prtype IN ('USAGE')
+          AND (pg_has_role(u_grantor.oid, 'USAGE')
+               OR pg_has_role(grantee.oid, 'USAGE')
+               OR grantee.rolname = 'PUBLIC')
+
+    UNION ALL
+
+    /* sequences */
+    SELECT CAST(u_grantor.rolname AS sql_identifier) AS grantor,
+           CAST(grantee.rolname AS sql_identifier) AS grantee,
+           CAST(current_database() AS sql_identifier) AS object_catalog,
+           CAST(n.nspname AS sql_identifier) AS object_schema,
+           CAST(c.relname AS sql_identifier) AS object_name,
+           CAST('SEQUENCE' AS character_data) AS object_type,
+           CAST('USAGE' AS character_data) AS privilege_type,
+           CAST(
+             CASE WHEN
+                  -- object owner always has grant options
+                  pg_has_role(grantee.oid, c.relowner, 'USAGE')
+                  OR c.grantable
+                  THEN 'YES' ELSE 'NO' END AS yes_or_no) AS is_grantable
+
+    FROM (
+            SELECT oid, relname, relnamespace, relkind, relowner, (aclexplode(coalesce(relacl, acldefault('r', relowner)))).* FROM pg_class
+         ) AS c (oid, relname, relnamespace, relkind, relowner, grantor, grantee, prtype, grantable),
+         pg_namespace n,
+         pg_authid u_grantor,
+         (
+           SELECT oid, rolname FROM pg_authid
+           UNION ALL
+           SELECT 0::oid, 'PUBLIC'
+         ) AS grantee (oid, rolname)
+
+    WHERE c.relnamespace = n.oid
+          AND c.relkind = 'S'
+          AND c.grantee = grantee.oid
+          AND c.grantor = u_grantor.oid
+          AND c.prtype IN ('USAGE')
           AND (pg_has_role(u_grantor.oid, 'USAGE')
                OR pg_has_role(grantee.oid, 'USAGE')
                OR grantee.rolname = 'PUBLIC');