Re: two queryes in a single tablescan

Stefano Dal Pra wrote:
> suppose you have a large table tab and two (or more) queryes like this:
>
> SELECT count(*),A FROM tab WHERE C GROUP BY A;
> SELECT count(*),B FROM tab WHERE C GROUP BY B;
>
> is there any way to get both results in a single query,
> eventually through stored procedure?
> The retrieved [count(*),A] ; [count(*),B)] data couldnt fit
> on a single table, of course.
>
> The main goal would be to get multiple results while scanning the
> table[s] once only
> thus getting results in a faster way.
>
> This seems to me quite a common situation but i have no clue whether a neat
> solution can be implemented through stored procedure.

With a temp table:

CREATE TEMPORARY TABLE tmp AS SELECT COUNT(*) as rows, a,b FROM WHERE C
GROUP BY a,b;
SELECT SUM(rows), a FROM tmp GROUP BY a;
SELECT SUM(rows), b FROM tmp GROUP BY b;
DROP TABLE tmp;

(Using temp tables in plpgsql procedures doesn't quite work until 8.3.
But you can use dynamic EXECUTE as a work-around. There used to be a FAQ
entry about that, but apparently it's been removed because the problem
has been fixed in the upcoming release.)

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com