From: matthias schoeneich <matthias.schoeneich@gmx.net>
Subject: Re: Efficiency in Multiple Queries of Same Table in a PL/PgSQL
	Function
Date: Sat, 17 Oct 2009 13:29:32 +0200
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	<4AD93837.7030500@gmail.com>
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To: pgsql-sql@postgresql.org

Hi,

as you don't seem to need the sigma_*'s, you could calc the whole result 
with one query using:

CREATE OR REPLACE FUNCTION poly_example2() RETURNS SETOF FLOAT8 AS
$poly_example$
DECLARE
     f_result   FLOAT8 := 0.0;
     i_rowcount INT    := 0  ;

BEGIN
   SELECT sum((RANDOM() * 100 ) * (term.i * term.i) + RANDOM()     * 
(term.j * term.j) + term.k) +
          sum((RANDOM() *  53 ) * (term.i * term.i) +(RANDOM()* 5) * 
(term.j * term.j) + term.k) +
          sum(    96.232234     * (term.i * term.i) + 0.32322325   * 
(term.j * term.j) + term.k) ,
          count(*)
     INTO f_result  ,
          i_rowcount
    FROM blah AS term;

   IF i_rowcount > 0 THEN
     RETURN NEXT f_result;
   ELSE
     RETURN NEXT 0;
   END IF;
END;
$poly_example$ LANGUAGE plpgsql STRICT IMMUTABLE;

I've just put it in your plpgsql body to handle the case where table 
blah contains no rows.

Matthias

Rob Sargent schrieb:
> I don't see anything in the assignment statements (sigma_* :=) which 
> would prevent one from doing all three of them within a single for 
> loop.  In fact, written as is there's some chance the values of the 
> sigma_*s might change between repeated calls to the function since there 
> is no explicit ordering of the rows returned from table blah.  Putting 
> all the assignments into a single select from blah would at least say 
> that the sigma values are from the same dataset per run.
> 
> 
> As to efficiency in general, I would expect the entire  table (~50 rows) 
> would be entirely in memory after the first select, but you plan triples 
> the time in the loop.  This expense would likely only be noticeable if 
> the function itself is called /lots/.
> 
> Gary Chambers wrote:
>> All...
>>
>> In the poly_example function below, I am emulating an actual
>> requirement by querying the same table three (3) times in order to
>> derive a solution to a problem.  Is this the best or most efficient
>> and effective way to implement this?  The table (which consists of
>> only five (5) FLOAT8 columns) I'm querying contains less than 50 rows.
>>  Thanks in advance for any insight or criticisms you offer.
>>
>> CREATE OR REPLACE FUNCTION poly_example() RETURNS SETOF FLOAT8 AS
>> $poly_example$
>> DECLARE
>>     term blah%ROWTYPE;
>>     sigma_l FLOAT8 := 0.0;
>>     sigma_b FLOAT8 := 0.0;
>>     sigma_r FLOAT8 := 0.0;
>>
>> BEGIN
>>     FOR term in SELECT * FROM blah LOOP
>>         sigma_l := sigma_l + (RANDOM() * 100) * (term.i * term.i) +
>>                 RANDOM() * (term.j * term.j) + term.k;
>>     END LOOP;
>>
>>     FOR term in SELECT * FROM blah LOOP
>>         sigma_b := sigma_b + (RANDOM() * 53) * (term.i * term.i) +
>>                 (RANDOM() * 5) * (term.j * term.j) + term.k;
>>     END LOOP;
>>
>>     FOR term in SELECT * FROM blah LOOP
>>         sigma_r := sigma_r + 96.232234 * (term.i * term.i) +
>>                 0.32322325 * (term.j * term.j) + term.k;
>>     END LOOP;
>>
>>     RETURN NEXT sigma_l + sigma_b + sigma_r;
>> END;
>> $poly_example$ LANGUAGE plpgsql STRICT IMMUTABLE;
>>
>> -- Gary Chambers
>>
>> /* Nothing fancy and nothing Microsoft! */
>>
>>   
> 
>