Re: Performance differences 7.1 to 7.3 - Mailing list pgsql-general

I think my indexes are OK.

I have no settings for 'shared memory buffers' in postgresql.conf on either system.  Both systems have same, i.e.
#sort_mem = 512
#shared_buffers = 2*max_connections # min 16
#fsync = true

Do you, or any of you, have any other suggestions for why such a simple "select count(*) from tpv;" should take six times as long on our latest and greatest dual processor server with almost the latest and greatest postgresql?  View tpv is a three table join which takes 3.38 seconds on the 'old' system and 18.09 seconds on the 'new' system.

Jim Apsey
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Dann Corbit wrote:

-----Original Message-----
From: Jimmie H. Apsey [mailto:japsey@futuredental.com] 
Sent: Tuesday, December 14, 2004 1:18 PM
To: Dann Corbit
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Performance differences 7.1 to 7.3

On the 'old' Red Hat AS 2.1 here is the results of explain and the
query:

[ ~]$ time /usr/bin/psql mpt -c"explain select count(*) from tpv;"
NOTICE:  QUERY PLAN:

Aggregate  (cost=4563.87..4563.87 rows=1 width=56) ->  Nested Loop  (cost=870.92..4563.01 rows=342 width=56)       ->  Hash Join  (cost=870.92..3869.17 rows=342 width=44)             ->  Seq Scan on treatment_plan  (cost=0.00..956.66 
rows=14844 width=28)             ->  Hash  (cost=602.33..602.33 rows=24033 width=16)                   ->  Seq Scan on treatment_plan_header  
(cost=0.00..602.33 rows=24033 width=16)       ->  Index Scan using ada_code_pkey on ada_code  (cost=0.00..2.01

rows=1 width=12)

EXPLAIN
0.010u 0.000s 0:00.03 33.3%    0+0k 0+0io 332pf+0w
[~]$ time /usr/bin/psql mpt -c"select count(*) from tpv;"count
-------33439
(1 row)

0.010u 0.000s 0:03.10 0.3%    0+0k 0+0io 332pf+0w
[japsey@DCFRAID ~]$

On the 'new' system:

[ ~]$  /usr/bin/psql mpt -c"explain select count(*) from tpv;"                                                       QUERY 
PLAN                                                
------------------------------------------------------------------------
---------------------------------------------------Aggregate  (cost=202529.15..202529.15 rows=1 width=45)  ->  Nested Loop  (cost=6262.46..202496.78 rows=12948 width=45)        Join Filter: ("outer".service_code = ("inner".ada_code)::text)        ->  Merge Join  (cost=6262.46..6754.54 rows=12933 width=36)              Merge Cond: (("outer".appointment_order = 
"inner".appointment_order) AND ("outer".pat_id = "inner".pat_id))              ->  Sort  (cost=2335.37..2395.35 rows=23992 width=14)                    Sort Key: treatment_plan_header.appointment_order, 
treatment_plan_header.pat_id                    ->  Seq Scan on treatment_plan_header  
(cost=0.00..589.92 rows=23992 width=14)              ->  Sort  (cost=3927.09..4016.27 rows=35672 width=22)                    Sort Key: treatment_plan.appointment_order, 
treatment_plan.pat_id                    ->  Seq Scan on treatment_plan  (cost=0.00..800.60 
rows=35672 width=22)                          Filter: (amount IS NULL)        ->  Seq Scan on ada_code  (cost=0.00..10.06 rows=406 width=9)
(13 rows)
[ ~]$

Where do I go to get clues about the results of "explain"? 

The command syntax:
http://www.postgresql.org/docs/current/static/sql-explain.html
A brief explanation:
http://www.freebsddiary.org/postgresql-analyze.php
A longer explanation:
http://www.postgresql.org/docs/7.4/interactive/performance-tips.html
A nuts and bolts look at how PostgreSQL performs queries:
http://candle.pha.pa.us/main/writings/pgsql/performance.pdf

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