"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.