Re: Window Functions

On Tuesday 14 October 2008 18:19:07 Hannu Krosing wrote:
> On Tue, 2008-10-14 at 11:05 +0200, Andreas Joseph Krogh wrote:
> > Hi all.
> > This is not very "hackers"-related, but related to the topic of window-funcitons, which seems to be discussed quite
abit on "hackers" these days. 
> >
> > Can window-functions in PG be used to return "total number of rows" in a "paged result"?
> > Say you have:
> > SELECT p.id, p.firstname
> >   FROM person p
> >  ORDER BY p.firstname ASC
> >  LIMIT 10 OFFSET 10
> >
> > Is it possible to use some window-function to return the "total-number of columns" in a separate column?
> >
> > In Oracle one can do
> > SELECT q.*, max(rownum) over() as total_rows FROM (subquery)
> > which returns the total number or columns in a separate column. This is very handy for web-pages which for example
needto display the rist 20 results of several million, without having to do a separate count(*) query. 
>
> no need to use window functions here, just ask for max inline:
>
>
> hannu=# select rownum, word, (select max(rownum) from words) as maxrow
> from words limit 10;
>  rownum |   word    | maxrow
> --------+-----------+--------
>       1 |           |  98569
>       2 | A         |  98569
>       3 | A's       |  98569
>       4 | AOL       |  98569
>       5 | AOL's     |  98569
>       6 | Aachen    |  98569
>       7 | Aachen's  |  98569
>       8 | Aaliyah   |  98569
>       9 | Aaliyah's |  98569
>      10 | Aaron     |  98569
> (10 rows)

Where do you get your "rownum"-column from here? It's a pseudo-column in Oracle which is computed for each row in the
"result-set",it's not a column in a table somewhere, which is why I figured I must use window-funciton, or "analytical
function"as Oracle calls them, to operate on the *result-set* to retrieve the maximum number of rows which satisfies
thequery. 

As far as I understand the ROW_NUMBER() window-funciton can be used to construct "limit with offset"-queries in a
SQL-spec-compliantway. 

Say I want to retrieve an ordered list of persons (by name):

SELECT * FROM (
SELECT ROW_NUMBER() OVER (order by p.name) as rnum, q.* FROM (
SELECT p.id, p.name FROM person p where p.birth_date > '2000-01-01'
) q
) rWHERE r.rnum between 11 AND 20
;

This is good and works in Oracle, PG >= 8.4 and others that implements spec-compliant window-functions. This is fine,
butin Oracle I can extend this query to this for getting the total-number (not just the "page" 11-20) of persons
matchingin a separate column: 

SELECT * FROM (
SELECT ROW_NUMBER() OVER (order by p.name) as rnum, q.*, max(rownum) over() as total_rows FROM (
SELECT p.id, p.name FROM person p where p.birth_date > '2000-01-01'
) q
) rWHERE r.rnum between 11 AND 20
;

So my question is: Will PG, with window functions, provide a similar mechanism for retrieving the total number of rows
inthe "result-set" without actually retrieving them all? I understand that PG might have to visit them all in order to
retrievethat count, but that's OK. 

What I'm looking for is an elegant solution to what's becomming a more common requirement in web-applications these
days:To display pageable lists with a "total-count", and to do that with *one* query, preferrably using
standard-compliantSQL. 

--
Andreas Joseph Krogh <andreak@officenet.no>
Senior Software Developer / CEO
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