On Mon, Sep 28, 2009 at 4:43 AM, Kouber Saparev wrote:
> Hello,
>
> I am using PostgreSQL 8.3.7 and I am experiencing an issue similar to the
> one I've already described some time ago:
> http://archives.postgresql.org/pgsql-performance/2009-02/msg00261.php
>
> Again, adding a LIMIT clause to a qu
Now I am experiencing similar issue with another table, called
"message", for which there's a conditional index:
CREATE TABLE message (
message_sid SERIAL PRIMARY KEY,
from_profile_sid INT NOT NULL REFERENCES profile,
to_profile_sid INT NOT NULL REFERENCES profile,
sender_has_deleted BOO
marcin mank writes:
> I hit an interestinhg paper on n_distinct calculation:
> http://www.pittsburgh.intel-research.net/people/gibbons/papers/distinct-values-chapter.pdf
I don't think we're quite ready to make ANALYZE read every row of a
table in order to estimate n_distinct. It is an interestin
> I hit an interestinhg paper on n_distinct calculation:
>
> http://www.pittsburgh.intel-research.net/people/gibbons/papers/distinct-values-chapter.pdf
>
> the PCSA algorithm described there requires O(1) calculation per
> value. Page 22 describes what to do with updates streams.
>
> This I think (
> So the bottom line here is just that the estimated n_distinct is too
> low. We've seen before that the equation we use tends to do that more
> often than not. I doubt that consistently erring on the high side would
> be better though :-(. Estimating n_distinct from a limited sample of
> the po
Kouber Saparev writes:
> Tom Lane wrote:
>> Hmph, that's still not real good. Ideally it should be estimating
>> *less* than the average frequency, because the estimate is made after
>> excluding all the most-common-values, which evidently 'kouber' is not
>> one of.
> I altered the statistics fo
Tom Lane wrote:
Kouber Saparev writes:
Now the planner believes there're 910 rows, which is a bit closer to the
real data:
swing=# select avg(length) from (select username, count(*) as length
from login_attempt group by username) as freq;
avg
--
491.608731042
Tom Lane wrote:
Robert Haas writes:
If you left seq_page_cost (which isn't mentioned here) at the default
value but reduced random_page_cost to 0.1, then you have
random_page_cost < seq_page_cost. That's probably Bad.
... well, it's certainly going to push the planner to believe indexscans
a
Kouber Saparev writes:
> Now the planner believes there're 910 rows, which is a bit closer to the
> real data:
> swing=# select avg(length) from (select username, count(*) as length
> from login_attempt group by username) as freq;
> avg
> --
> 491.608731042755547
Richard Huxton wrote:
Since it's expecting 7914 rows for "kouber" it thinks it will find the
20 rows you want fairly quickly by just looking backward through the
login_attempt_pkey index.
Try increasing the stats on the username column.
ALTER TABLE login_attempt ALTER COLUMN username SET STATIS
Robert Haas writes:
> If you left seq_page_cost (which isn't mentioned here) at the default
> value but reduced random_page_cost to 0.1, then you have
> random_page_cost < seq_page_cost. That's probably Bad.
... well, it's certainly going to push the planner to believe indexscans
are cheaper tha
On Mon, Feb 23, 2009 at 7:26 AM, Kouber Saparev wrote:
> Now, recently I have altered some of the default parameters in order to get
> as much as possible out of the hardware - 12 GB of RAM, 8 processors. So, I
> guess I have done something wrong, thus the planner is taking that wrong
> decision.
Kouber Saparev wrote:
> db=# EXPLAIN ANALYZE
> SELECT
> *
> FROM
> login_attempt
> WHERE
> username='kouber'
> ORDER BY
> login_attempt_sid DESC;
>
> QUERY PLAN
> --
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