How to fetch values at regular hours?

Goran Hasse wrote:

Hello all!

I am logging my electricity meeter every five minute!

But the problem is that I would like to get values *just before* every
hour - like

select * from counter_log_view where name='CNT-3' and timestamp <
'2010-05-23 22:00:00' order by timestamp desc limit 1;
name | timestamp | count
-------+----------------------------+-------
CNT-3 | 2010-05-23 21:53:20.844422 | 69
(1 rad)

But I don't want to do a lot of selections and gather all the data in
a file.
Is there any better way to create the query so that I get the values
just before
every hour?

wild guess, untested.. something like...

select * from counter_log_view where name = 'CNT-3' and extract(minute
from timestamp) >= 55 order by timestamp;

Goran Hasse <gorhas@gmail.com> wrote:

[...]
I would like to do;

freescada=> select * from counter_log_view where name='CNT-3' and timestamp
< '2010-05-23 18:00:00' order by timestamp desc limit 1;
name | timestamp | count
-------+---------------------------+-------
CNT-3 | 2010-05-23 17:53:18.58674 | 43
(1 rad)

freescada=> select * from counter_log_view where name='CNT-3' and timestamp
< '2010-05-23 19:00:00' order by timestamp desc limit 1;
name | timestamp | count
-------+----------------------------+-------
CNT-3 | 2010-05-23 18:53:19.151988 | 50
(1 rad)

freescada=> select * from counter_log_view where name='CNT-3' and timestamp
< '2010-05-23 20:00:00' order by timestamp desc limit 1;
name | timestamp | count
-------+----------------------------+-------
CNT-3 | 2010-05-23 19:53:19.683514 | 51
(1 rad)

In one query. Is this possible in *any* way?

Sure:

| SELECT DISTINCT ON (DATE_TRUNC('hour', timestamp)) name, timestamp, count
| FROM counter_log_view
| ORDER BY DATE_TRUNC('hour', timestamp), timestamp DESC;

Tim

P. S.: Naming columns "timestamp" and "count" will lead to
trouble :-).

Yes timestamp and count - is not good names for columns...

I tried something like;

select name,date_trunc('hour',timestamp),timestamp,count from
counter_log_view where name='CNT-3' and timestamp < '2010-05-23 20:00:00'
order by timestamp limit 10;
name | date_trunc | timestamp | count
-------+---------------------+----------------------------+-------
CNT-3 | 2010-05-23 15:00:00 | 2010-05-23 15:43:17.411386 | 23
CNT-3 | 2010-05-23 15:00:00 | 2010-05-23 15:53:17.45934 | 24
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:03:17.489321 | 24
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:13:17.586089 | 24
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:23:17.69116 | 25
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:33:17.795955 | 28
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:43:17.89265 | 28
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:53:17.989268 | 30
CNT-3 | 2010-05-23 17:00:00 | 2010-05-23 17:03:18.1447 | 33
CNT-3 | 2010-05-23 17:00:00 | 2010-05-23 17:13:18.199568 | 35
(10 rader)

Seems promising... But then I would like to select only the last from
date_trunc...

Hm...

GH

2010/5/25 Tim Landscheidt <tim@tim-landscheidt.de>

Goran Hasse <gorhas@gmail.com> wrote:

[...]
I would like to do;

freescada=> select * from counter_log_view where name='CNT-3' and

timestamp

< '2010-05-23 18:00:00' order by timestamp desc limit 1;
name | timestamp | count
-------+---------------------------+-------
CNT-3 | 2010-05-23 17:53:18.58674 | 43
(1 rad)

freescada=> select * from counter_log_view where name='CNT-3' and

timestamp

< '2010-05-23 19:00:00' order by timestamp desc limit 1;
name | timestamp | count
-------+----------------------------+-------
CNT-3 | 2010-05-23 18:53:19.151988 | 50
(1 rad)

freescada=> select * from counter_log_view where name='CNT-3' and

timestamp

< '2010-05-23 20:00:00' order by timestamp desc limit 1;
name | timestamp | count
-------+----------------------------+-------
CNT-3 | 2010-05-23 19:53:19.683514 | 51
(1 rad)

In one query. Is this possible in *any* way?

Sure:

| SELECT DISTINCT ON (DATE_TRUNC('hour', timestamp)) name, timestamp, count
| FROM counter_log_view
| ORDER BY DATE_TRUNC('hour', timestamp), timestamp DESC;

Tim

P. S.: Naming columns "timestamp" and "count" will lead to
trouble :-).

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gorhas@gmail.com
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Goran Hasse <gorhas@gmail.com> wrote:

Yes timestamp and count - is not good names for columns...

I tried something like;

select name,date_trunc('hour',timestamp),timestamp,count from
counter_log_view where name='CNT-3' and timestamp < '2010-05-23 20:00:00'
order by timestamp limit 10;
name | date_trunc | timestamp | count
-------+---------------------+----------------------------+-------
CNT-3 | 2010-05-23 15:00:00 | 2010-05-23 15:43:17.411386 | 23
CNT-3 | 2010-05-23 15:00:00 | 2010-05-23 15:53:17.45934 | 24
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:03:17.489321 | 24
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:13:17.586089 | 24
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:23:17.69116 | 25
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:33:17.795955 | 28
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:43:17.89265 | 28
CNT-3 | 2010-05-23 16:00:00 | 2010-05-23 16:53:17.989268 | 30
CNT-3 | 2010-05-23 17:00:00 | 2010-05-23 17:03:18.1447 | 33
CNT-3 | 2010-05-23 17:00:00 | 2010-05-23 17:13:18.199568 | 35
(10 rader)

Seems promising... But then I would like to select only the last from
date_trunc...

Hm...

Why did you not use the query I posted:

[...]

Sure:

| SELECT DISTINCT ON (DATE_TRUNC('hour', timestamp)) name, timestamp, count
| FROM counter_log_view
| ORDER BY DATE_TRUNC('hour', timestamp), timestamp DESC;

[...]

Is copy & paste too much effort?

Tim