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 a bit 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 need to 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 the query.

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

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
) r
WHERE 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, but in Oracle I can extend this query to this for getting the total-number (not just the "page" 11-20) of persons matching in 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
) r
WHERE 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 in the "result-set" without actually retrieving them all? I understand that PG might have to visit them all in order to retrieve that 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-compliant SQL.

--
Andreas Joseph Krogh <andreak(at)officenet(dot)no>
Senior Software Developer / CEO
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