Re: 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