W dniu 01/26/2013 02:49 PM, Alban Hertroys pisze:
On Jan 26, 2013, at 13:32, Rafał Pietrak <rafal@xxxxxxxxxxxxxxxxxx> wrote:
I have a usage recording table: CREATE TABLE readings(tm timestamp, bytesin int, bytesout int);
The readouts are made "occasionally" - the timespan between the readouts are not very precise, but there is a lot of those readouts.
when presenting data, for every readout I need to compute the difference between it and the one immediately preceding it, and divide that by respective measurement interval.
You don't need a self-join, you need a window function.
See: http://www.postgresql.org/docs/9.1/static/tutorial-window.html
For example:
SELECT timestamp, lag(timestamp) OVER (ORDER BY timestamp) FROM readings;
Yes. That's what I needed (another example, of how fragmented knowledge
of a subject makes one (myself) use quite inapropriate keywords when
searching).
Thenx,
-R
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