Re: Difference in columns

On Sun, May 11, 2008 at 11:47 PM, Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>
wrote:

> Mag Gam wrote:
>
> >
> > ts | size| Diff
> > -------------------+-----+------
> > 2002-03-16 | 11 | 0
> >
> > 2002-03-17 | 15 | 4
> > 2002-03-18 | 18 | 3
> > 2002-03-19 | 12 | -6
> >
> >
> > I need the first column to be 0, since it will be 11-11. The second
> > colum is 15-11. The third column is 18-15. The fourth column is 12-18.
> >
> > Any thoughts about this?
>
> Without making any comments on the advisability of the structure you're
> trying to use, here are a few ideas.
>
> The easy way is to use PL/PgSQL and FOR EACH .. SELECT . It's probably
> going to be rather fast too as it can use a single sequential scan.
>
> Otherwise (all examples use the following code):
>
> CREATE TABLE x (ts timestamp, size int);
> INSERT INTO x (ts, size) VALUES
> ('2002-03-16',11),
> ('2002-03-17',15),
> ('2002-03-18',18),
> ('2002-03-19',12);
>
> If you can assume that there is always exactly 1 day between entries
> then it's easy enough with a self join.
>
> If you cannot assume that, you can use a subquery with limit and order
> by to obtain the next record:
>
> SELECT
> a.ts,
> (SELECT b.size FROM x b WHERE b.ts > a.ts ORDER BY b.ts ASC LIMIT 1)
> - a.size AS difference
> FROM x a;
>
> ... but that'll be really slow for any significant number of entries.

not really... if you have an index on the TS column.

Best regards,
--
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