The comment of Primary key

Hi Dave.

I am distracted!
Though a command is psql, it is the problem that it happens in pgAdmin3.

See this sample below.
---------------------------------------------------------------
CREATE TABLE "MyTransaction"
(
  "XID" int4 NOT NULL,
  "LastSeqId" int4 NOT NULL,
  "HostId" int4 NOT NULL,
  CONSTRAINT "MyTransaction_pkey" PRIMARY KEY ("XID", "HostId")
) WITH OIDS;
COMMENT ON CONSTRAINT "MyTransaction_pkey" ON "MyTransaction" IS 'What happens to this?';

----------------------------------------------------------------
saito=# SELECT cls.oid, cls.relname as idxname, indrelid, indkey, indisclustered, indisunique, indisprimary, n.nspname,
       indnatts, tab.relname as tabname, indclass, description,
       pg_get_expr(indpred, indrelid, true) as indconstraint, contype, condeferrable, condeferred, amname
  FROM pg_index idx
  JOIN pg_class cls ON cls.oid=indexrelid
  JOIN pg_class tab ON tab.oid=indrelid
  JOIN pg_namespace n ON n.oid=tab.relnamespace
  JOIN pg_am am ON am.oid=cls.relam
  LEFT OUTER JOIN pg_description des ON (des.objoid=cls.oid AND des.objsubid = 0)
  LEFT OUTER JOIN pg_constraint con ON con.conrelid=indrelid AND conname=cls.relname
 WHERE cls.relname = 'MyTransaction_pkey';

  oid   |      idxname       | indrelid | indkey | indisclustered | indisunique | indisprimary | nspname | indnatts |
tabname 
| indclass  | description | indconstraint | contype | condeferrable | condeferred | amname

--------+--------------------+----------+--------+----------------+-------------+--------------+---------+----------+-----------
----+-----------+-------------+---------------+---------+---------------+-------------+--------
 518874 | MyTransaction_pkey |   518872 | 1 3    | f              | t           | t            | saito   |        2 |
MyTransaction | 1978 1978 |             |               | p       | f             | f           | btree
(1 row)

*Why?*

saito=# SELECT * FROM pg_description WHERE objoid >= 518874::OID;
 objoid | classoid | objsubid |      description
--------+----------+----------+-----------------------
 518875 |    16386 |        0 | What happens to this?
(1 row)

Hmm... 518875?
Do I have misunderstanding?

Regards,
Hiroshi Saito