Re: Result set granularity..

In article <59410(dot)193(dot)243(dot)134(dot)6(dot)1064668289(dot)squirrel(at)www(dot)defero(dot)se>,
"Rasmus Aveskogh" <rasmus(at)defero(dot)se> writes:

> 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?

You could fake a rownum by means of a sequence:

CREATE SEQUENCE tmpseq;

SELECT <column>
FROM (
SELECT nextval ('tmpseq') AS rownum, <column>
FROM <table>
) AS <table>_with_rownum
WHERE rownum % 5 = 0;

DROP SEQUENCE tmpseq;