Re: performance penalty between Postgresql 8.3.8 and 8.4.1

Hi Andres,

This query returns for 8.4.1 and for 8.3.8 the same result:

stadistinct = -1
stanullfrac = 0
stawidth = 4
array_upper nothing

Regards

David

>-----Ursprüngliche Nachricht-----
>Von: Robert Haas [mailto:robertmhaas(at)gmail(dot)com]
>Gesendet: Dienstag, 8. Dezember 2009 05:05
>An: Kevin Grittner
>Cc: Schmitz, David; pgsql-performance(at)postgresql(dot)org
>Betreff: Re: [PERFORM] performance penalty between Postgresql
>8.3.8 and 8.4.1
>
>On Mon, Dec 7, 2009 at 5:19 PM, Kevin Grittner
><Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
>> "Schmitz, David" <david(dot)schmitz(at)harman(dot)com> wrote:
>>
>>> It is carried out with poor performance on postgresql 8.4.1 However
>>> postgresql 8.3.8 performs just fine.
>>> If you take a closer look at the query with EXPLAIN, it becomes
>>> obvious, that postgresql 8.4 does not consider the primary key at
>>> level 3 and instead generates a hash join:
>>
>>> Postgresql 8.4.1:
>>>
>>> Sort (cost=129346.71..129498.64 rows=60772 width=61)
>>
>>> Postgresql 8.3.8:
>>>
>>> Sort (cost=3792.75..3792.95 rows=81 width=61)
>>
>> It determines the plan based on available statistics, which in this
>> case seem to indicate rather different data. Do the two databases
>> have identical data? Have they both been recently analyzed?
> What is
>> the default_statistics_target on each? Do any columns in
>these tables
>> have overrides?
>
>I think Tom made some changes to the join selectivity code
>which might be relevant here, though I'm not sure exactly
>what's going on. Can we see, on the 8.4.1 database:
>
>SELECT SUM(1) FROM rdf_admin_hierarchy;
>SELECT s.stadistinct, s.stanullfrac, s.stawidth,
>array_upper(s.stanumbers1, 1) FROM pg_statistic s WHERE
>s.starelid = 'rdf_admin_hierarchy'::regclass AND s.staattnum =
>(SELECT a.attnum FROM pg_attribute a WHERE a.attname =
>'admin_place_id' AND a.attrelid = 'rdf_admin_hierarchy'::regclass);
>
>...Robert
>

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