User check

PostgreSQL,

I want to definitively check whether a user is logged in. I see there
is a table pg_stat_activity:

ben=# SELECT DISTINCT datname FROM ps_stat_activity

Are these better ways, or is this about right?

If right, does this table ever become corrupt, eg a TCP connection
failure without termination?

Whilst I am writing, something which has been bothering me. Does any
person know the quickest method for checking the existence of a record,
selected by Primary Key?

ben=# SELECT pk FROM table WHERE pk = value

I guess this way might be performance impared as it has to create the
output table with the extracted value.

ben=# SELECT COUNT(*) FROM table WHERE pk = value

I guess this might be faster, since it's only counting results without
creating a table. Or it might be slower if the table has already been
created, and then counted...

Ever a Novice, Ben.