simple query with radically different plan after 9.0 -> 9.2 upgrade

[Kevin Goess <kgoess@xxxxxxxxxxx>]

Given this table

articles_1=> \d hits_user_daily_count;
      Table "public.hits_user_daily_count"
          Column           |  Type   | Modifiers 
---------------------------+---------+-----------
 userid                    | integer | not null
 date                      | date    | not null
 num_hits                  | integer | default 0
 num_cover_page_hits       | integer | default 0
 num_additional_files_hits | integer | default 0
Indexes:
    "hits_user_daily_count_pkey" PRIMARY KEY, btree (userid, date)
    "hits_user_daily_count_date" btree (date)

whose data looks like this:

articles_1=> select * from hits_user_daily_count limit 5;
 userid  |    date    | num_hits | num_cover_page_hits | num_additional_files_hits 
---------+------------+----------+---------------------+---------------------------
 1590185 | 2013-07-10 |        3 |                   4 |                         0
  391907 | 2013-07-10 |       16 |                  12 |                         0
 1739541 | 2013-08-03 |        1 |                   0 |                         0
 1798435 | 2013-07-10 |        0 |                   1 |                         0
 1521468 | 2013-07-10 |        2 |                   0 |                         0

We noticed a big change after upgrading from 9.0 to 9.2. For yesterday‘s date, the query plan is fine, like you’d expect

articles_1=> explain (analyze, buffers) select 1
        from hits_user_daily_count
        where userid = 1590185 and date = '2013-07-30';
                                                                      QUERY PLAN                                              
------------------------------------------------------------------------------------------------------------------------------
 Index Only Scan using hits_user_daily_count_pkey on hits_user_daily_count  (cost=0.00..8.02 rows=1 width=0) (actual time=0.02
   Index Cond: ((userid = 1590185) AND (date = '2013-07-30'::date))
   Heap Fetches: 1
   Buffers: shared hit=5
 Total runtime: 0.044 ms

but for today‘s date it looks like it’s reading all the rows for that date, 15,277 buffers:

articles_1=> explain (analyze, buffers) select 1
        from hits_user_daily_count
        where userid = 1590185 and date = '2013-08-01';
                                                                     QUERY PLAN                                               
------------------------------------------------------------------------------------------------------------------------------
 Index Scan using hits_user_daily_count_date on hits_user_daily_count  (cost=0.00..7.92 rows=1 width=0) (actual time=11.957..1
   Index Cond: (date = '2013-08-01'::date)
   Filter: (userid = 1590185)
   Rows Removed by Filter: 20149
   Buffers: shared hit=15277
 Total runtime: 17.924 ms

(The dates in the queries are old because I've had this email in draft for a while, but the behavior is still identical).

We‘ve addressed the problem by running ’analyze' on the table every day ate about 1:30am. Buffer hits on that table go from about 1,000/sec to 70,000/sec between midnight and that analyze job, and then go back down to 1,000/sec and stay flat until midnight rolls around again.

Before the 9.0 -> 9.2 upgrade, the behavior would be flat all day.

Any ideas what would be causing that problem?