Sequential scan where Index scan expected. - Mailing list pgsql-sql

I'm getting sequential scans (and poor performance), on scans using my
primary keys.
Can anyone help figure out why?

demo=# \d xx_thing                    Table "public.xx_thing"        Column          |            Type             |
Modifiers
-------------------------+-----------------------------+-----------thing_id              | bigint
|not null
 
thing_model           | character varying(128)      |thing_color           | character varying(128)      |thing_year
       | integer                     |
 
Indexes:   "xx_thing_pkey" primary key, btree (thing_id)


demo=# analyze verbose xx_thing_event;
INFO:  analyzing "public.xx_thing_event"
INFO:  "xx_thing_event": 3374 pages, 3000 rows sampled, 197478 estimated
total rows


demo=# explain update xx_thing_event set thing_color='foo' where
thing_event_id=10000;                            QUERY PLAN
---------------------------------------------------------------------Seq Scan on xx_thing_event  (cost=0.00..5842.48
rows=1width=110)  Filter: (thing_event_id = 10000)
 
(2 rows)