[ADMIN] is a user in a group?

[Jacopo Silva]

I need to find out a quick way to check which groups a user
is member of.

In the pg_group table each group row has an array of
userids of the members of the group.
At the moment I am selecting all the rows than
searching in each array for the id of the user
I am looking for.

There should be something more elegant than this!!!
There should be a system table which I can use to find
the relation between user and groups.

I would be very grateful to anyone who can help me:-).

Bye,
Jacopo