Weiwei Guo <wwguocn@gmail.com> writes:
> Yes, I set a password. I'm sure that I used the right password for
> 'postgres'. I can su to 'postgres' with it.
That would be your Unix password for the Unix account 'postgres'.
This has approximately zero to do with either the Postgres username
'postgres', or the Postgres password for that username.
regards, tom lane