how could select id=xx so slow?

I have grabbed one day slow query log and analyzed it by pgfouine, to my
surprise, the slowest query is just a simple select statement:

*select diggcontent_data_message.thing_id, diggcontent_data_message.KEY,
diggcontent_data_message.value, diggcontent_data_message.kind FROM
diggcontent_data_message WHERE diggcontent_data_message.thing_id = 3570882;*

where thing_id is the primary key, guess how long it takes?

754.61 seconds!!

I tried explain analyze it and below is the result, which is very fast:

*
*
*
explain analyze select diggcontent_data_message.thing_id,
diggcontent_data_message.KEY, diggcontent_data_message.value,
diggcontent_data_message.kind FROM diggcontent_data_message WHERE
diggcontent_data_message.thing_id = 3570882;

QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using idx_thing_id_diggcontent_data_message on
diggcontent_data_message (cost=0.00..15.34 rows=32 width=51) (actual
time=0.080..0.096 rows=8 loops=1)
Index Cond: (thing_id = 3570882)
Total runtime: 0.115 ms
(3 rows)
*

so I wonder could this simple select is innocent and affected badly by
other queries? how could I find those queries that really slow down the
database?
thanks!