Subject: Re: libpq: How are result sets fetched behind the scene?
To: pgsql-sql@lists.postgresql.org
References: <6f2f210b-53e6-f350-4850-83f49b2a1354@4js.com>
From: Sebastien FLAESCH <sf@4js.com>
Organization: Four Js Development Tools
Message-ID: <7e0c3606-1513-1120-555a-6a14b615d939@4js.com>
Date: Thu, 29 Aug 2019 18:39:13 +0200
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Sorry must read "fetched" in title.

Similar question:

When execution a SELECT (returning a large result set):

Is the whole result set fetched to the client app, not matter what row number is provided to the first PQgetvalue() call (or similar API call on 
result set data or meta-data)?

Seb

On 8/29/19 6:32 PM, Sebastien FLAESCH wrote:
> Hi all,
> 
> We are using libpq in our C program.
> 
> I was wondering how the client lib / protocol manage to fetch the result set to the client app memory.
> 
> Is there some optimization ?
> 
> Are rows fetched in sequence until the requested row?
> 
> I mean if I do a SELECT that produces a huge result set, and then call PQgetvalue() with a row_number = 5000, are all 5000 rows fetched from the 
> server or is there some smart fast-forward done because I did not ask the 4999 previous rows?
> 
> We know about server cursors (declare + fetch forward), and we do already fetch blocks of rows with this.
> 
> But server cursors are slow (and certainly use more server resources), compared to direct SELECT execution.
> 
> So we are wondering what would be the impact in terms of resources on the client application.
> 
> Is there any doc link or blog that would explain best practices with libpq result set programming?
> 
> Thanks!
> Seb
> 
>