Worse perfomance on 8.2.0 than on 7.4.14 - Mailing list pgsql-performance

Hi

I have a simple query which uses 32ms on 7.4.14 and 1015ms on 8.2.0.
I guess 7.4.14 creates a better execution plan than 8.2.0 for this query but
i don't know how to get it to select a better one.
Explain analyse output will be found near the end of the e-mail.

(I have simplified my real query to get it as simple as possible. The original query
contain 6 tables and was acceptable on 7.4.2, but took far too long on 8.1.4)

I have made a test setup to compare 7.4.14, 8.1.4 and 8.2.0.
8.1.4 and 8.2.0 uses the same execution plan and same time to execute.

postgresql.conf values i changed is
7.4.14
    Raised shared_buffers from 32MB to 128MB
    Raised temp_buffers from 8MB to 32MB
8.2.0
    Raised shared_buffers from 32MB to 128MB
    Raised temp_buffers from 8MB to 32MB
    Raised work_mem from 1MB to 8MB

(It did however not have any influence of speed for
the view_subset query shown below.)

vacuum analyze has been executed.

Computer:
    Dell PowerEdge 2950
    openSUSE Linux 10.1
    Intel(R) Xeon 3.00GHz
    4GB memory
    xfs filesystem on SAS disks

 Table "public.step_result_subset"
   Column    |  Type   | Modifiers
-------------+---------+-----------
 id          | integer | not null
 uut_result  | integer |
 step_parent | integer |
Indexes:
    "step_result_subset_pkey" PRIMARY KEY, btree (id)
    "step_result_subset_parent_key" btree (step_parent)
    "step_result_uut_result_idx" btree (uut_result)
Table contain 17 179 506 rows, and is ~400M when exported to file

             Table "public.uut_result_subset"
     Column      |            Type             | Modifiers
-----------------+-----------------------------+-----------
 id              | integer                     | not null
 start_date_time | timestamp without time zone |
Indexes:
    "uut_result_subset_pkey" PRIMARY KEY, btree (id)
    "uut_result_subset_start_date_time_idx" btree (start_date_time)
Table contain ~176 555 rows, and is ~4.7M when exportd to file

Query is defined as view:

create view view_subset as
select
  ur.id as ur_id,
  sr.id as sr_id
from
  uut_result_subset as ur
   inner join step_result_subset as sr
    on ur.id=sr.uut_result
where
  ur.start_date_time > '2006-12-11'
  and sr.step_parent=0;

Explain analyze is run several times to get a stable result
so i guess the numbers presented is with as much as possible
data in memory buffers.

Column step_result_subset.step_parent contain 0 in as many rows as there are rows in table uut_result_subset.
(In my data set this will be 176 500 rows, Other values for step_result_subset.step_parent is present 1003 times and
lower.)

Query: "select * from view_subset;" run against 7.4.14 server.
QUERY PLAN
------------------------------------------------------------------------
 Nested Loop  (cost=0.00..1400.86 rows=17 width=8) (actual time=0.161..26.287 rows=68 loops=1)
   ->  Index Scan using uut_result_subset_start_date_time_idx on uut_result_subset ur  (cost=0.00..63.28 rows=18
width=4)(actual time=0.052..0.195 rows=68 loops=1) 
         Index Cond: (start_date_time > '2006-12-11 00:00:00'::timestamp without time zone)
   ->  Index Scan using step_result_uut_result_idx on step_result_subset sr  (cost=0.00..74.28 rows=2 width=8) (actual
time=0.149..0.379rows=1 loops=68) 
         Index Cond: ("outer".id = sr.uut_result)
         Filter: (step_parent = 0)
 Total runtime: 26.379 ms

Query: "select * from view_subset;" run against 8.4.0 server.

QUERY PLAN
----------------------------------------------------------------------
 Hash Join  (cost=339.61..77103.61 rows=96 width=8) (actual time=5.249..1010.669 rows=68 loops=1)
   Hash Cond: (sr.uut_result = ur.id)
   ->  Index Scan using step_result_subset_parent_key on step_result_subset sr  (cost=0.00..76047.23 rows=143163
width=8)(actual time=0.082..905.326 rows=176449 loops=1) 
         Index Cond: (step_parent = 0)
   ->  Hash  (cost=339.31..339.31 rows=118 width=4) (actual time=0.149..0.149 rows=68 loops=1)
         ->  Bitmap Heap Scan on uut_result_subset ur  (cost=4.90..339.31 rows=118 width=4) (actual time=0.060..0.099
rows=68loops=1) 
               Recheck Cond: (start_date_time > '2006-12-11 00:00:00'::timestamp without time zone)
               ->  Bitmap Index Scan on uut_result_subset_start_date_time_idx  (cost=0.00..4.90 rows=118 width=0)
(actualtime=0.050..0.050 rows=68 loops=1) 
                     Index Cond: (start_date_time > '2006-12-11 00:00:00'::timestamp without time zone)
 Total runtime: 1010.775 ms

Thanks for tips.

Best regards
Rolf Østvik