nbeweb=> EXPLAIN select to_char(created, 'IW') as week, count(session_id) from session WHERE username IS NULL and drus(created) = '2002' group by week ORDER BY week;
NOTICE:  QUERY PLAN:

Aggregate  (cost=92.62..92.74 rows=2 width=154)
  ->  Group  (cost=92.62..92.68 rows=22 width=154)
        ->  Sort  (cost=92.62..92.62 rows=22 width=154)
              ->  Index Scan using session_u_idx on session  (cost=0.00..92.12 rows=22 width=154)

EXPLAIN
nbeweb=> EXPLAIN ANALYZE select to_char(created, 'IW') as week, count(session_id) from session WHERE username IS NULL and drus(created) = '2002' group by week ORDER BY week;
NOTICE:  QUERY PLAN:

Aggregate  (cost=92.62..92.74 rows=2 width=154) (actual time=764457.99..837660.30 rows=36 loops=1)
  ->  Group  (cost=92.62..92.68 rows=22 width=154) (actual time=763934.48..830755.34 rows=846053 loops=1)
        ->  Sort  (cost=92.62..92.62 rows=22 width=154) (actual time=763934.45..798906.16 rows=846053 loops=1)
              ->  Index Scan using session_u_idx on session  (cost=0.00..92.12 rows=22 width=154) (actual time=140.71..253768.07 rows=846053 loops=1)
Total runtime: 837759.64 msec

EXPLAIN
nbeweb=>