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

-----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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