Need some help in postgres locking mechanism

Hi
I have two problems to discuss.an you please guide me how to proceed on
this.
*problem 1:*
I have table X('a' is prmiary key) and table Y('b' is primary key).
Table Y has ''a''(X primary key) as foreign key.I have one job which runs
once in one hour.I want to lock the rows in table Y.but this will also lock
table X rows to avoid concurrent actions.My table X is used by other jobs
as well for updation.
The solution what I thought is implement KEY SHARE on 'a' column in table
Y.That will help others jobs to update the corresponding rows in table X
without any issue.

My doubt is *how to keep key share lock on "a" column in table Y*.by
default lock is on column 'b' which is primary key of table Y. My table X
is so huge.I dont want any other locks on it.
we are using postgres 9.2.

*one more doubt:**If I implement key share lock on 'a' column,will the
default locks on table X and table Y removed or not?*

*PLEASE PROVIDE SYNTAX AS WELL*

*problem2:*
This is a different issue.I have a table Q('w' is primary key).When a job
runs ,i want to lock some rows so that the other parallel job wont be
considering this row.

what is the simple and best lock I can implement on these rows?I want with
NOWAIT option.

kindly give solutions to above issues.I would be greatful for that.

Thanks
K.Santhosh