On 20 May 2010 17:36, David Jarvis <thangalin@gmail.com> wrote:
> Hi, Thom.
>
> The query is given two items:
>
> Range of years
> Range of days
>
> I need to select all data between the range of days (e.g., Dec 22 - Mar 22)
> over the range of years (e.g., 1950 - 1970), such as shown here:
>
> http://i.imgur.com/MUkuZ.png
>
> For Jun 1 to Jul 1 it would be no problem because they the same year. But
> for Dec 22 to Mar 22, it is difficult because Mar 22 is in the next year
> (relative to Dec 22).
>
> How do I do that without strings?
>
> Dave
>
>
Okay, get your app to convert the month-date to a day of year, so we
have year_start, year_end, day_of_year_start, day_of_year_end
and your where clause would something like this:
WHERE extract(YEAR from m.taken) BETWEEN year1 and year2
AND (
extract(DOY from m.taken) BETWEEN day_of_year_start AND day_of_year_end
OR (
extract(DOY from m.taken) >= day_of_year_start OR extract(DOY from
m.taken) <= day_of_year_end
)
)
... substituting the placeholders where they appear.
So if we had:
year1=1941
year2=1952
day_of_year_start=244 (based on input date of 1st September)
day_of_year_end=94 (based on 4th April)
We'd have:
WHERE extract(YEAR from m.taken) BETWEEN 1941 and 1952
AND (
extract(DOY from m.taken) BETWEEN 244 AND 94
OR (
extract(DOY from m.taken) >= 244 OR extract(DOY from m.taken) <= 94
)
)
Then you could add expression indexes for the YEAR and DOY extract parts, like:
CREATE INDEX idx_taken_doy ON climate.measurement (EXTRACT(DOY from taken));
CREATE INDEX idx_taken_year ON climate.measurement (EXTRACT(YEAR from taken));
Although maybe you don't need those, depending on how the date
datatype matching works in the planner with the EXTRACT function.
Regards
Thom
