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Re: is parallel union all possible over dblink?

From: Greg Spiegelberg <gspiegelberg(at)gmail(dot)com>
To: Svetlin Manavski <svetlin(dot)manavski(at)gmail(dot)com>
Cc: Marinos Yannikos <mjy(at)geizhals(dot)at>, pgsql-performance(at)postgresql(dot)org
Subject: Re: is parallel union all possible over dblink?
Date: 2011-06-30 12:37:24
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Lists: pgsql-performance
On Thu, Jun 30, 2011 at 3:02 AM, Svetlin Manavski <
svetlin(dot)manavski(at)gmail(dot)com> wrote:

> I am now a bit puzzled after the initial satisfaction by Marinos' reply.
> 1. what do you mean exactly by "to ensure your UNION succeeds". The dblink
> docs do not mention anything about issues using directly  the suggested
> dblink_send_query() + dblink_get_results(). What problems should I expect in
> using them as suggested by Marinos?
Admittedly, I hadn't used those specific dblink functions and imagined
dblink_get_result() failing if the query on the connection wasn't finished.
 It appears now that after some experimentation that it's perfectly happy
hanging until the query is finished executing.

> 2. If I understand correctly your method, it is not applicable from inside
> a stored procedure, is it? I need to keep all the business logic within
> PostgreSQL and provide just a clean interface to a simple GUI layer
Then dblink is your answer.  My suggestion applies if you were implementing
a solution in the application.

> 3. Unfortunately GridSQL and Pgpool-II do not seem mature and stable
> products to be used in commercial software. Neither one provides clear
> documentation. GridSQL has been discontinued and it is not clear what kind
> of future it will have. I have not tried GridSQL but I did try Pgpool-II. It
> is disappointing that it may stop working correctly even just because of the
> way you write the query (e.g. using uppercase in a field or using named
> field in group by, ecc.). Even worse, when it does not recognize something
> in the parallel query, it just provides incorrect result (from only the
> local DB) rather than raising an exception. So I guess Pgpool-II in its
> current state is good only for very simple applications, which are not
> supposed to be reliable at all.
I don't think GridSQL is discontinued.  Appears though EnterpriseDB has open
sourced it and moved to  Not
incredibly active but some as recent as last month.

Sorry for the confusion.


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