table-level and row-level locks.

Iam trying to acquire rowlevel locks in postgresql. I try doing this:

 'select * from students where name='Larry' for update;

But by looking at the holding array of proclock , I've noticed that by doing this only

AccessShareLock gets acquired which is a table level lock.

How do I acquire rowlevelock and what fields of Lock or Proclock datastructures indicate it.

Thanks

Jenny