again on index usage (7.1.3)

I had one more frustrating exprience with the 7.1.3 optimizer handling
index/scan selection.

Here is the schema

RADIUS=# \d attrib
Table "attrib"
Attribute | Type | Modifier
-----------+----------------+---------------------
user_name | character(32) | not null default ''
attr | character(32) | not null default ''
value | character(128) |
op | character(2) |
Index: uattr

RADIUS=# \d uattr
Index "uattr"
Attribute | Type
-----------+---------------
user_name | character(32)
attr | character(32)
op | character(2)
btree

(this is for use by gnu-radius).

RADIUS=# select count(*) from attrib;
count
--------
396117
(1 row)

RADIUS=# select count(distinct user_name) from attrib;
count
-------
62713
(1 row)

each username has more or less the same number of attributes.

SELECT * FROM attrib WHERE user_name = 'xyz';

always results in sequential scan.

As you can see, there is sufficient number of different user_name values - why
the sequential scan?

Needless to say that turning off sequential scans results is measurably faster
index scan.

Daniel