From: Artur Zając <azajac@ang.com.pl>
To: pgsql-performance@postgresql.org
Subject: Speed of EXCECUTE in PL/PGSQL
Date: Thu, 14 Mar 2013 20:22:07 +0100
Message-ID: <035b01ce20e9$38ec92a0$aac5b7e0$@ang.com.pl> (raw)
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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
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To: pgsql-performance@postgresql.org
Cc: azajac@ang.com.pl
Subject: Re: Speed of EXCECUTE in PL/PGSQL
In-Reply-To: <035b01ce20e9$38ec92a0$aac5b7e0$@ang.com.pl>
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