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From: Shridhar Daithankar <shridhar_daithankar@persistent.co.in>
To: pgsql-general@postgresql.org
Subject: Re: Result set granularity..
Date: Mon, 29 Sep 2003 13:12:00 +0530
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On Saturday 27 September 2003 18:41, Rasmus Aveskogh wrote:
> Hi,
>
> Since I went from Oracle to PostgreSQL I've been missing the "invisable"
> column 'rownum'. I often used it to lower the granularity of my data.
> For example, say I have a large table containing some sort of statistical
> data and want to plot a graph using it.
> If the graph is 600 pixels wide I might as well lower the granularity of
> my incoming data to 600 measure points before plotting.
>
> In Oracle I used to do this by using the modulus operator on the rownum
> column as a restriction.
>
> SELECT <column> FROM <table> WHERE mod(rownum, 5) = 0;
>
> The query above would give me every fifth row of the original result set
> and would save me from sending the data over my database connection and do
> the lowering of the granularity in the application.
>
> I have two questions, one dependent on the answer on the other one..
>
> 1) Is it possible to achieve this any other _easy_ way?
> Perhaps it would be feasible to write a wrapper using a counter which
> makes the SELECT and then return every fifth row to the SELECT calling the
> wrapper. But then I assume the data still has to "travel" one step which
> puts on some overhead.

First of all there is oid which you can use but it is not strictly unique. 
It's limited to 4GB and wraps around.

I would rather say declare a cursor and move the pointer as and when required. 
Sice you aren't retrieving large part of rows, that loop should be cheap 
enough.

HTH

 Shridhar