On Thu, 25 Aug 2005, Aldor wrote:
As I know EXPLAIN ANALYZE runs the query. I think you are just running the
query two times. The first time you run the query it will take a long time to
be processed - after the first run the query planner will remember the best
way to run the query so your second run runs much faster.
I can reproduce this behavior for some queries under 8.0.1 - so I'm not 100%
sure if it is the same behavior under 7.4.2.
I'm still wondering why you first query takes about 4107.987 ms - this kind
of query has usually have to run much much faster.
That would work if I were to get really occasional high values in syslog,
but this is almost a consist thing over a very short period of time:
Aug 25 14:53:32 forgehouse-s1 postgres[23721]: [2-1] LOG: duration: 567.559 ms
statement: UPDATE session SET hit_time=now() WHERE
md5='7537b74eab488de54d6e0167d1919207';
Aug 25 14:53:32 forgehouse-s1 postgres[23722]: [2-1] LOG: duration: 565.966 ms
statement: UPDATE session SET hit_time=now() WHERE
md5='d84613009a95296fb511c2cb051ad618';
Aug 25 14:53:33 forgehouse-s1 postgres[23723]: [2-1] LOG: duration: 1192.789
ms statement: UPDATE session SET hit_time=now() WHERE
md5='d84613009a95296fb511c2cb051ad618';
Aug 25 14:53:53 forgehouse-s1 postgres[23727]: [2-1] LOG: duration: 12159.162
ms statement: UPDATE session SET hit_time=now() WHERE
md5='d84613009a95296fb511c2cb051ad618';
Aug 25 14:53:54 forgehouse-s1 postgres[23728]: [2-1] LOG: duration: 3283.185
ms statement: UPDATE session SET hit_time=now() WHERE
md5='7537b74eab488de54d6e0167d1919207';
Aug 25 14:53:57 forgehouse-s1 postgres[23729]: [2-1] LOG: duration: 2116.516
ms statement: UPDATE session SET hit_time=now() WHERE
md5='7537b74eab488de54d6e0167d1919207';
And you will notice that the last two are the exact same md5 value ...
When did you vacuumed the table the last time?
pg_autovacuum is running to keep things up to date, and I just ran a
VACUUM FULL *and* a REINDEX, just to make sure things are clean ...
No matter when I try it, I can't seem to get a value above 10ms for that
query above when I do it from psql ...
Marc G. Fournier wrote:
'k, I've been wracking my brains over this today, and I'm either
mis-understanding what is being reported *or* its reporting wrong ...
According to syslog:
LOG: duration: 4107.987 ms statement: UPDATE session SET hit_time=now()
WHERE md5='8b8e7b7ff9b1b2ed5fc60218ced28d00';
But, if I do an EXPLAIN ANALYZE:
# explain analyze UPDATE session SET hit_time=now() WHERE
md5='702c6cb20d5eb254c3feb2991e7e5e31';
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------
Index Scan using session_md5_key on "session" (cost=0.00..6.01 rows=1
width=93) (actual time=0.060..0.060 rows=0 loops=1)
Index Cond: (md5 = '702c6cb20d5eb254c3feb2991e7e5e31'::bpchar)
Total runtime: 0.171 ms
(3 rows)
And it doesn't matter what value I put for md5, I still get <1ms ...
I could see some variations, but almost 4000x slower to *really* run the
query vs an explain analyze?
This is with 7.4.2 ...
----
Marc G. Fournier Hub.Org Networking Services (http://www.hub.org)
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Marc G. Fournier Hub.Org Networking Services (http://www.hub.org)
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