first_value refers to the first row from the window frame. Unless you force some kind of ordering, you cannot expect
consistent results out of this.
See the PARTITION BY ... ORDER BY syntax in http://www.postgresql.org/docs/9.1/static/tutorial-window.html
On Δευ 20 Αυγ 2012 01:55:38 Thalis Kalfigkopoulos wrote:
Sorry for the lack of a more appropriate title.
The summary of my problem is: i run a query and I get some results; then I create a view using this query, and I run the same query on the view, and get different results. Details follow.
On the original table the analytical data is as follows:
# SELECT id,experiment,insertedon,score FROM data WHERE id=1160;
id | experiment | insertedon | score
--------+----------------------+---------------------+--------
1160 | alpha | 2012-08-19 01:01:12 | 220.69
1160 | alpha | 2012-08-19 01:01:22 | 220.69
1160 | beta | 2012-08-19 01:01:31 | 220.7
1160 | beta | 2012-08-19 01:01:42 | 220.7
1160 | beta | 2012-08-19 01:01:54 | 220.7
My query of interest using windowing functions is:
# SELECT id, experiment, first_value(insertedon) OVER (PARTITION BY score, id) AS first_insertedon, score FROM data WHERE id=1160;
id | experiment | first_insertedon | score |
--------+----------------------+---------------------+--------+----------
1160 | alpha | 2012-08-19 01:01:12 | 220.69 |
1160 | alpha | 2012-08-19 01:01:12 | 220.69 |
1160 | beta | 2012-08-19 01:01:31 | 220.7 |
1160 | beta | 2012-08-19 01:01:31 | 220.7 |
1160 | beta | 2012-08-19 01:01:31 | 220.7 |
So far so good. I then create the view on this last query without the WHERE condition:
# CREATE VIEW clustered_view AS SELECT id, experiment, first_value(insertedon) OVER (PARTITION BY score, id) AS first_insertedon, score FROM data;
I see the view created correctly and its definition is according to the mentioned SQL query. I now select from the view adding the WHERE condition:
# SELECT * from clustered_view WHERE id=1160;
id | experiment | first_insertedon | score |
--------+----------------------+---------------------+--------+
1160 | alpha | 2012-08-19 01:01:12 | 220.69 |
1160 | alpha | 2012-08-19 01:01:12 | 220.69 |
1160 | beta | 2012-08-19 01:01:54 | 220.7 |
1160 | beta | 2012-08-19 01:01:54 | 220.7 |
1160 | beta | 2012-08-19 01:01:54 | 220.7 |
As you see, the 'first_insertedon' timestamp for the experiment 'beta' is no longer the first of the timestamps i.e. '2012-08-19 01:01:31' as the original query's results gave correctly, but it's now the last one i.e. '2012-08-19 01:01:54'
Any ideas? Missing the obvious?
TIA,
Thalis K.
-
Achilleas Mantzios
IT DEPT
