get SYSID of current user as var for query

Hi I`m nearly new to Postgres and like to do something like that in a
query:

select * from testtab where user=SYSID;

Ilike to use the SYSID of current user as var for the query.

The reson:
Sometimes I have to build a table whith a "createdby"-Stamp. Until all
persons in a company never leave or change there job thats no
problem...but when they do I have to update all tables where I used
"createdby=user". If I coud use the ID I just change the name of the
user instad of creating a new one.

cu Simon