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
