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Worse perfomance on 8.2.0 than on 7.4.14

From: Rolf Østvik <rolfostvik(at)yahoo(dot)no>
To: pgsql-performance(at)postgresql(dot)org
Subject: Worse perfomance on 8.2.0 than on 7.4.14
Date: 2006-12-31 11:33:09
Message-ID: 200612311233.10201.rolfostvik@yahoo.no (view raw or flat)
Thread:
Lists: 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..0.379 rows=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=68 loops=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) (actual time=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

Responses

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Next:From: Claus GuttesenDate: 2006-12-31 12:16:43
Subject: Re: Worse perfomance on 8.2.0 than on 7.4.14
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Subject: Re: High update activity, PostgreSQL vs BigDBMS

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