Re: EXPLAIN ANALYZE on 8.2

Evgeny Gridasov <eugrid(at)fpm(dot)kubsu(dot)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