take my index, please

greetings, data mutilators,

postgres 7.1, redhat 6.1

I've read a few recent threads about the planner not choosing the
indexes people would like, but my situation seems extraordinary
because I don't even get an index scan on the PRIMARY KEY, even when I
set enable_seqscan to off!

The query is as simple as select a from b where c = n;

I would be very grateful if anyone could explain (heh) to me why the
sequential scan is preferred because this exercise is not academic.

here are some details:

webco=# \d allocation
                  Table "allocation"
   Attribute    |           Type           | Modifier
----------------+--------------------------+----------
 allocation_oid | bigint                   | not null
 state          | character varying        |
 location_oid   | bigint                   |
 agent_oid      | bigint                   |
 patron_oid     | bigint                   |
 creation_time  | timestamp with time zone |
 pickup_time    | timestamp with time zone |
 return_time    | timestamp with time zone |
 summary        | character varying        |
 schedule_rule  | character varying        |
 resource_rule  | character varying        |
Index: allocationpk

webco=# \d allocationpk
  Index "allocationpk"
   Attribute    |  Type
----------------+--------
 allocation_oid | bigint
unique btree

webco=# explain select * from allocation where allocation_oid = 5;
NOTICE:  QUERY PLAN:

Seq Scan on allocation  (cost=0.00..2295.79 rows=1 width=104)

EXPLAIN
webco=# set enable_seqscan TO OFF;
SET VARIABLE
webco=# explain select * from allocation where allocation_oid = 5;
NOTICE:  QUERY PLAN:

Seq Scan on allocation  (cost=100000000.00..100002295.79 rows=1 width=104)

EXPLAIN

indeed.

-Lyn