RE: Wrong FOR UPDATE lock type - Mailing list pgsql-hackers

>     I'm  about  99.666667% sure that the lock type choosen in the
>     FOR UPDATE case (line  511  of  parse_relation.c)  should  be
>     RowExclusiveLock  instead  of  RowShareLock.   Actually I get
>     "Deadlock risk" debug messages when selecting FOR UPDATE  and
>     then really UPDATE.

http://www.postgresql.org/users-lounge/docs/6.5/user/x3116.htm

RowShareLock
Acquired by SELECT FOR UPDATE and LOCK TABLE for IN ROW SHARE MODE
statements. 

Conflicts with ExclusiveLock and AccessExclusiveLock modes. 

Vadim