Re: hash aggregation

"IOS scan" ?
Index Only Scan

What's your seq_page_cost and random_page_cost?

hashes=# SELECT name, setting, reset_val FROM pg_settings WHERE setting <> reset_val;
          name           |    setting     | reset_val
-------------------------+----------------+-----------
 archive_command         | (disabled)     |
 enable_bitmapscan       | off            | on
 enable_indexscan        | off            | on
 enable_seqscan          | off            | on
 log_file_mode           | 0600           | 384
 random_page_cost        | 0.1            | 4
 seq_page_cost           | 0.1            | 1
 transaction_isolation   | read committed | default
 unix_socket_permissions | 0777           | 511
(9 rows)

Postgresql 9.2.1 was configured and built with default settings.

Thank you.