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From: Claudio Freire <klaussfreire@gmail.com>
Date: Thu, 24 Aug 2017 13:14:05 -0300
Message-ID: 
 <CAGTBQpb_V9UymUr0ZARY2y0P_s+7DozKoTUK-pok==o5vx4XsA@mail.gmail.com>
Subject: Re: performance problem on big tables
To: Mariel Cherkassky <mariel.cherkassky@gmail.com>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
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On Thu, Aug 24, 2017 at 4:51 AM, Mariel Cherkassky
<mariel.cherkassky@gmail.com> wrote:
> Hi Claudio, how can I do that ? Can you explain me what is this option ?
>
> 2017-08-24 2:15 GMT+03:00 Claudio Freire <klaussfreire@gmail.com>:
>>
>> On Mon, Aug 21, 2017 at 5:00 AM, Mariel Cherkassky
>> <mariel.cherkassky@gmail.com> wrote:
>> > To summarize, I still have performance problems. My current situation :
>> >
>> > I'm trying to copy the data of many tables in the oracle database into
>> > my
>> > postgresql tables. I'm doing so by running insert into
>> > local_postgresql_temp
>> > select * from remote_oracle_table. The performance of this operation are
>> > very slow and I tried to check the reason for that and mybe choose a
>> > different alternative.
>> >
>> > 1)First method - Insert into local_postgresql_table select * from
>> > remote_oracle_table this generated total disk write of 7 M/s and actual
>> > disk
>> > write of 4 M/s(iotop). For 32G table it took me 2 hours and 30 minutes.
>> >
>> > 2)second method - copy (select * from oracle_remote_table) to /tmp/dump
>> > generates total disk write of 4 M/s and actuval disk write of 100 K/s.
>> > The
>> > copy utility suppose to be very fast but it seems very slow.
>>
>> Have you tried increasing the prefetch option in the remote table?
>>
>> If you left it in its default, latency could be hurting your ability
>> to saturate the network.
>
>

Please don't top-post.

I'm assuming you're using this: http://laurenz.github.io/oracle_fdw/

If you check the docs, you'll see this:
https://github.com/laurenz/oracle_fdw#foreign-table-options

So I'm guessing you could:

ALTER FOREIGN TABLE remote_table OPTIONS ( SET prefetch 10240 );


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