How can I determine if a table exists in the current search_path with PLPGSQL? I'm writing a setup script for an application that's an addon for another application, so I want to check if the tables for the other application exist.  If not, I want to give the user a useful error.  However, I don't know what schema will be holding the tables.

DO LANGUAGE plpgsql $$
BEGIN
    PERFORM 1
    FROM
        pg_catalog.pg_class c
        JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
    WHERE
        n.nspname = current_setting('search_path')
        AND c.relname = 'foo'
        AND c.relkind = 'r'; -- not sure if I actually need this or not...

    IF NOT FOUND THEN
        RAISE 'This application depends on tables created by another application';
    END IF;
END;
$$;


However, current_setting('search_path') returns a TEXT containing "$user",public by default, which isn't terribly useful.

The only other thing I can think of is to try selecting from the table and catch the exception.  It would do the job, but I don't think it is very elegant and I've read that it is expensive to use (though maybe that would be ok in this scenario since I'm only running it once?).
Quick and dirty

In Postgres 9.4+ use 

SELECT to_regclass('foo');


Returns NULL if the identifier is not found in the search path.
In Postgres 9.3 or older use a cast to regclass:

SELECT 'foo'::regclass;


This raises an exception, if the object is not found!

If 'foo' is found, the oid is returned in its text representation. That's just the table name, schema-qualified according to the current search path and double-quoted where necessary.

If the object is not found you can be sure it does not exist anywhere in the search path - or not at all for a schema-qualified name (schema.foo). 

If it's found there are two shortcomings:


The search includes implicit schemas of the search_path, namely pg_catalog and pg_temp. But you may want to exclude temp and system tables for your purpose. (?)
A cast to regclass works for all objects in the system catalog pg_class: indexes, views, sequences etc. Not just tables. You seem to be looking for a regular table exclusively. However, you'll probably have problems with other objects of the same name, too. Details:


How to check if a table exists in a given schema



Slow and sure

We are back to your query, but don't use current_setting('search_path'), which returns the bare setting. Use the dedicated system information function current_schemas(). Per documentation:


  current_schemas(boolean) name[]
  names of schemas in search path, optionally including implicit schemas


"$user" in the search path is resolved smartly. If no schema with the name of SESSION_USER exists, the schema is not returned to begin with. Also, depending on what you want exactly, you can additionally output implicit schemas (pg_catalog and possibly pg_temp) - but I assume you don't want those for the case at hand, so use:

DO 
$do$
BEGIN
   IF EXISTS (
      SELECT  -- list can be empty
      FROM   pg_catalog.pg_class c
      JOIN   pg_catalog.pg_namespace n ON n.oid = c.relnamespace
      WHERE  n.nspname = ANY(current_schemas(FALSE))
      AND    n.nspname NOT LIKE 'pg_%'  -- exclude system schemas!
      AND    c.relname = 'foo'
      AND    c.relkind = 'r')           -- you probably need this
   THEN
      RAISE 'This application depends on tables created by another application';
   END IF;
END
$do$;


SQL Fiddle, demonstrating all except for the last DO statement.
SQL Fiddle (JDBC) has problems with DO statements containing termination characters.