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Re: Same query, same performance

From: Tomasz Myrta <jasiek(at)klaster(dot)net>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Same query, same performance
Date: 2003-01-23 08:20:09
Message-ID: 3E2FA5B9.1030105@klaster.net (view raw or flat)
Thread:
Lists: pgsql-performance
alexandre :: aldeia digital wrote:

>Hi all,
>
>First, sorry for the long mail...
>
>I have a system with 7 Million of records in 600 tables.
>My actual production machine is: P4 1.6G, 3 IDE 7200, 1GB PC133
>My new machine production is: Dual Xeon 2.0G HT, 1GB DDR266 ECC
>3 SCSI with HW Raid 5
>
>The postgresql.conf is the SAME in both systems and I test
>with no other connections, only my local test.
>
>shared_buffers = 80000
>effective_cache_size = 60000
>random_page_cost = 2.5
>cpu_tuple_cost = 0.001
>cpu_index_tuple_cost = 0.0001
>cpu_operator_cost = 0.00025
>
>My question is:
>
>If I execute the same query executed a lot of times, the
>duration is praticaly the same in both systems ?
>
>1) !       1.185424 elapsed 1.090000 user 0.100000 system sec
>2) !       1.184415 elapsed 1.070000 user 0.120000 system sec
>3) !       1.185209 elapsed 1.100000 user 0.080000 system sec
>
>If the disks is not read directly, the system must find
>the rows in RAM. If it find in RAM, why so diffrents machines
>have the times of execution and why the times does not down ???

Here is your problem:
->  Seq Scan on fn06t t1  (cost=0.00..25808.30 rows=15 width=95)
               Filter: ((fn06emp07 = 1::smallint) AND (fn06tiptit =
'R'::bpchar) AND (fn06titban = '002021001525               
  '::bpchar))

Problably system has to read from disk whole table fn06t each time, beacuse it
doesn't use index scan.

Do you have any indexes on table fn06t? How selective are conditions above
How big is this table? Can you use indexes on multiple fields on this table 
- it should help, because conditions above return only 15 rows?

Regards,
Tomasz Myrta




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