Path to top of tree

Given a table which includes tree-type information consisting of an id
and a parent_id, is there an already existing function that will return
the path to the top of the tree for a given record?

The connectby function from the contrib tablefuncs does what I want for
a whole table, but I haven't found a way to execute it in an efficient
way to get the information for a single record. A query in the form of
"select connectby(.....) where ..." will return the answer I want, but
it builds the tree on the whole table and then filters to get the record
I want which, for 5000 records, is taking about half a minute.

Before I start writing my own function, have I overlooked something
already available or some better way to invoke connectby?

Cheers,
Steve