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Do parallel queries work with only dblink not with fdw? - Mailing list pgsql-general

From Klaus P.
Subject Do parallel queries work with only dblink not with fdw?
Date
Msg-id CAGZW8jUP61bf2bEdmgL827vq=Nt93MFE4YQqjHM=CCyKF95=Xg@mail.gmail.com
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Responses Re: Do parallel queries work with only dblink not with fdw?  (Adrian Klaver <adrian.klaver@aklaver.com>)
Re: Do parallel queries work with only dblink not with fdw?  (Marc Mamin <M.Mamin@intershop.de>)
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In order to improve cpu and disk utilization, I am testing parallel queries.

The approach with dblink_send_query() and dblink_get_result() works in my proof-of-concept. Runtime of my reference query was reduced from 55 seconds to ~20seconds using 4 parallel connections. Not what I had hoped but certainly a significant improvement.

My approach is fairly easy:

db_link_send_query('conn1', 'statement based on partitioning field');
db_link_send_query('conn2', 'statement based on partitioning field');

...

SELECT
dblink_get_result('conn1')
UNION ALL
dblink_get_result('conn2')

...


However, using fdw foreign data wrappers, I was not able to run any query in parallel. I came across this presentation http://de.slideshare.net/EnterpriseDB/foreign-data-wrappers-and-you-with-postgres where it says on page 12  "parallel FDW access". Is there any example available on how to run FDW queries in parallel?

My approach with FDW is similar to the dblink example above:

SELECT * FROM myFdwTable WHERE (clause based on partitioning field)
UNION ALL
SELECT * FROM myFdwTable WHERE (clause based on partitioning field)
...

My experience is however that the statements are carried out sequentially.

Thanks

Klaus Pieper

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