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Re: two queryes in a single tablescan

From: "Stefano Dal Pra" <s(dot)dalpra(at)gmail(dot)com>
To: "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>
Cc: pgsql-sql(at)postgresql(dot)org, pgsql-performance(at)postgresql(dot)org
Subject: Re: two queryes in a single tablescan
Date: 2007-10-17 13:21:55
Message-ID: 290a5abc0710170621o3e2534b1l91f3c0fcf93f532a@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performancepgsql-sql
On 10/17/07, Heikki Linnakangas <heikki(at)enterprisedb(dot)com> wrote:
> 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;
>

Thank You.

I actually already do something like that:
in a stored procedure i do create a md5 hash using passed parameters
converted to TEXT
and get a unix_like timestamp using now()::abstime::integer.
This gets me a string like: 9ffeb60e9e6581726f7f5027b42c7942_1192443215
which i do use to
EXECUTE
 CREATE TABLE 9ffeb60e9e6581726f7f5027b42c7942_1192443215 AS
    SELECT * FROM
getjd('''||param1||''','''||param2||''','||param3||','||param4||')'


The 9ffeb60e9e6581726f7f5027b42c7942_1192443215 is what i called 'tab'
in my first post,
and i need to perform about 7 queryes on that. (after a while i will
drop the table using the timestamp part of the name, but that's
another point).

Here is where i would like to scan once only that table. Depending on
parameters it may get as big as 50Mb (this actually is the tablespace
size growth) or more with about 10^6 tuples.

 Stefano


> (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
>

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