Re: Group by range in hour of day - Mailing list pgsql-general

On Mar 16, 2015, at 2:22 PM, David G. Johnston <david.g.johnston@gmail.com> wrote:

On Mon, Mar 16, 2015 at 3:13 PM, Adrian Klaver <adrian.klaver@aklaver.com> wrote:

On 03/16/2015 02:57 PM, Israel Brewster wrote:

I have a table with two timestamp columns for the start time and end
time of each record (call them start and end).I'm trying to figure out
if there is a way to group these records by "hour of day", that is the
record should be included in the group if the hour of the day for the
group falls anywhere in the range [start,end]. Obviously each record may
well fall into multiple groups under this scenario.

The goal here is to figure out, for each hour of the day, a) what is the
total number of "active" records for that hour, and b) what is the total
"active" time for those records during the hour, with an ultimate goal
of figuring out the average active time per record per hour.

So, for simplified example, if the table contained three records:

          start              |               end
-----------------------------------------------------
2015-03-15 08:15  |  2015-03-15 10:45
2015-03-15 09:30  |  2015-03-15 10:15
2015-03-15 10:30  |  2015-03-15 11:30


Then the results should break out something like this:

hour  |  count  |  sum
-----------------------------
8       |    1       |   0.75
9       |    2       |   1.5
10     |    3       |   1.5
11     |    1       |   0.5

I can then easily manipulate these values to get my ultimate goal of the
average, which would of course always be less than or equal to 1. Is
this doable in postgress? Or would it be a better idea to simply pull
the raw data and post-process in code? Thanks.

Do not have an answer for you, but a question:

What version of Postgres are you on?

This will help determine what tools are available to work with.

​The following will give you endpoints for your bounds.  Version is important since "range types" could be very useful in this situation - but you'd still need to generate the bounds info regardless.​

​

SELECT * 

FROM

(SELECT * FROM generate_series('2015-03-15'::timestamptz, '2015-03-16'::timestamptz, '1 hour'::interval) start (start_ts)) AS s

CROSS JOIN

(SELECT end_ts + '1 hour'::interval AS end_ts FROM generate_series('2015-03-15'::timestamptz, '2015-03-16'::timestamptz, '1 hour'::interval) e (end_ts)) AS e

You would join this using an ON condition with an OR (start BETWEEN [...] OR end BETWEEN [...]) - range logic will be better and you may want to adjust the upper bound by negative 1 (nano-second?) to allow for easier "<=" logic if using BETWEEN.

Thanks, that is very helpful, but are you sure CROSS JOIN is what you wanted here? using that, I get a 625 row result set where each row from the first SELECT is paired up with EVERY row from the second select. I would think I would want the first row of the first SELECT paired up with only the first row of the second, second row of the first paired with the second row of the second, etc - i.e. 24 start and end bounds. Or am I missing something?