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