From: Tom Lane <tgl@sss.pgh.pa.us>
To: Evgeny Gridasov <eugrid@fpm.kubsu.ru>
Cc: pgsql-performance@postgresql.org
Subject: Re: EXPLAIN ANALYZE on 8.2
Date: Thu, 14 Dec 2006 11:41:15 -0500
Message-ID: <21516.1166114475@sss.pgh.pa.us> (raw)
In-Reply-To: <20061214192920.c75385d6.eugrid@fpm.kubsu.ru>
References: <20061214163230.d46f0138.eugrid@fpm.kubsu.ru>
<21146.1166112702@sss.pgh.pa.us>
<20061214192920.c75385d6.eugrid@fpm.kubsu.ru>
Evgeny Gridasov <eugrid@fpm.kubsu.ru> writes:
> This is a Linux Debian 3.1 ontop of 2x XEON 3.4 Ghz.
> PostgreSQL is 8.2 checked out from CVS REL8_2_STABLE yesterday.
> I'm running the same Postgres on another machine,
> with Debian Etch and have the same results.
Hmph. With 8.2 on Fedora 5 on a 2.8Ghz dual Xeon, I get this:
regression=# create table foo as select x from generate_series(1,2500000) x;
SELECT
regression=# vacuum foo;
VACUUM
regression=# checkpoint;
CHECKPOINT
regression=# \timing
Timing is on.
regression=# select count(*) from foo;
count
---------
2500000
(1 row)
Time: 666.639 ms
regression=# select count(*) from foo;
count
---------
2500000
(1 row)
Time: 609.514 ms
regression=# explain analyze select count(*) from foo;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
Aggregate (cost=44764.00..44764.01 rows=1 width=0) (actual time=1344.812..1344.813 rows=1 loops=1)
-> Seq Scan on foo (cost=0.00..38514.00 rows=2500000 width=0) (actual time=0.031..748.571 rows=2500000 loops=1)
Total runtime: 1344.891 ms
(3 rows)
Time: 1345.755 ms
regression=# explain analyze select count(*) from foo;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
Aggregate (cost=44764.00..44764.01 rows=1 width=0) (actual time=1324.846..1324.847 rows=1 loops=1)
-> Seq Scan on foo (cost=0.00..38514.00 rows=2500000 width=0) (actual time=0.046..748.582 rows=2500000 loops=1)
Total runtime: 1324.902 ms
(3 rows)
Time: 1325.591 ms
regression=#
which works out to about 0.14 microsec per gettimeofday call, on a
machine that ought to be slower than yours. So I think you've got
either a crummy motherboard, or a kernel that doesn't know the best
way to read the clock on that hardware. There is some discussion
of this in the archives (probably in pgsql-hackers); look back around
May or so when we were unsuccessfully trying to hack EXPLAIN to use
fewer gettimeofday calls.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, eugrid@fpm.kubsu.ru
Subject: Re: EXPLAIN ANALYZE on 8.2
In-Reply-To: <21516.1166114475@sss.pgh.pa.us>
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