how could select id=xx so slow?

[Yan Chunlu <springrider@xxxxxxxxx>]

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!