Speed of EXCECUTE in PL/PGSQL

Hi,

I have PostgreSQL 9.0.12 on Windows.

I have some simple function:

CREATE OR REPLACE FUNCTION sfunction() RETURNS BOOL AS

$BODY$

DECLARE

q TEXT;

r RECORD;

BEGIN

q='SELECT 1 from tb_klient LIMIT 0';

FOR r IN EXECUTE q

LOOP

END LOOP;

RETURN NULL;

RETURN NULL;

END;

$BODY$

LANGUAGE 'plpgsql';

And some simple Query:

explain analyze SELECT sfunction() AS value

FROM (

SELECT 5604913 AS id ,5666 AS idtowmag

) AS c

LEFT OUTER JOIN tg_tm AS tm ON (tm.ttm_idtowmag=c.idtowmag);

When I run this query explain analyze is:

Subquery Scan on a (cost=0.00..0.27 rows=1 width=8) (actual
time=24.041..24.042 rows=1 loops=1)

-> Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.001..0.002
rows=1 loops=1)

"Total runtime: 24.068 ms"

But when I change:

1. Table tb_klient to some other table (but not any other - queries
with some tables are still slow) or

2. "FOR r IN EXECUTE q"
change to
"FOR r IN SELECT 1 from tb_klient LIMIT 0" or

3. add "LEFT OUTER JOIN tb_klient AS kl ON
(kl.k_idklienta=c.idtowmag)" to query

Explain analyze of query is:

"Subquery Scan on a (cost=0.00..0.27 rows=1 width=8) (actual
time=1.868..1.869 rows=1 loops=1)"

" -> Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.002..0.002
rows=1 loops=1)"

"Total runtime: 1.894 ms"

Explain analyze of "SELECT 1 from tb_klient LIMIT 0" is:

"Limit (cost=0.00..0.13 rows=1 width=0) (actual time=0.001..0.001 rows=0
loops=1)"

" -> Seq Scan on tb_klient (cost=0.00..854.23 rows=6823 width=0) (never
executed)"

"Total runtime: 0.025 ms"

tb_klient has 8200 rows and 77 cols.

Why speed of executing (or planning) some very simple query from string in
pl/pgsql is dependent from whole query or why "FOR r IN EXECUTE q" is
significally slower from "FOR r IN query"?

-------------------------------------------

Artur Zajac