Re: Strange query planner behavior

Hi Tomas

I tried the method of using IN instead of =, it's not really better in
the present case:

EFT_MBON=# explain analyse select * from livraison where id_master in
(select 10);
                                                          QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
 Nested Loop Semi Join  (cost=0.00..211841.54 rows=1919474 width=697)
(actual time=1245.271..1245.271 rows=0 loops=1)
   Join Filter: (livraison.id_master = (10))
   Rows Removed by Join Filter: 1921796
   ->  Seq Scan on livraison  (cost=0.00..201476.37 rows=1919474
width=697) (actual time=0.006..211.196 rows=1921796 loops=1)
   ->  Materialize  (cost=0.00..0.01 rows=1 width=4) (actual
time=0.000..0.000 rows=1 loops=1921796)
         ->  Result  (cost=0.00..0.01 rows=1 width=4) (actual
time=0.001..0.001 rows=1 loops=1)
 Planning Time: 0.200 ms
 Execution Time: 1245.309 ms
(8 lignes)

I tried many different query planner cost variables (including defaults)
but no impact, as expected (default for random_page_cost is 4 so 15 is
about 3-4 times the default)
This was a very precise case of a special data distribution, so we'll go
for a workaround on this one

Best regards and thanks
Martin

On 30/11/2019 15:53, Tomas Vondra wrote:
> On Sat, Nov 30, 2019 at 11:29:53AM +0100, EffiSYS / Martin Querleu wrote:
>> Hi Pavel
>>
>> Thanks for the fast reply
>> Our databases are VACUUMed everyday. I did it again but no difference
>>
>> Here are the query plans:
>>
>> EFT_MBON=# explain analyse select * from livraison where id_master = 10;
>>  QUERY PLAN
>> --------------------------------------------------------------------------------------------------------------------------------------------
>>
>>  Index Scan using pour_recherche_sous_livraison on livraison
>>  (cost=0.03..15.04 rows=1 width=697) (actual time=0.017..0.017 rows=0
>> loops=1)
>>    Index Cond: (id_master = 10)
>>  Planning Time: 0.124 ms
>>  Execution Time: 0.036 ms
>> (4 lignes)
>>
>> EFT_MBON=# explain analyse select * from livraison where id_master =
>> (select 10);
>>                                                       QUERY PLAN
>> -----------------------------------------------------------------------------------------------------------------------
>>
>>  Seq Scan on livraison  (cost=0.01..2888156.69 rows=1917632
>> width=697) (actual time=1334.615..1334.615 rows=0 loops=1)
>>    Filter: (id_master = $0)
>>    Rows Removed by Filter: 1918196
>>    InitPlan 1 (returns $0)
>>      ->  Result  (cost=0.00..0.01 rows=1 width=4) (actual
>> time=0.000..0.001 rows=1 loops=1)
>>  Planning Time: 0.138 ms
>>  Execution Time: 1334.642 ms
>> (7 lignes)
>>
>> Regarding the cost calculator the configuration is as follows:
>>
>> random_page_cost and seq_page_cost are identical since the data is
>> 100% in RAM (both at 15.0, 3 times default)
>
> Ummm, what? Does this mean you have
> random_page_cost = 15
> seq_page_cost = 15
>
> Neither of that is 3 times the default value, though, so maybe I just
> don't understand correctly.
>
> regards
>

--
Martin Querleu - Directeur Général
EffiSYS (www.effitrace.fr - www.logistique-e-commerce.fr)
martin(dot)querleu(at)effisys(dot)fr
3, rue Gustave Delory
59000 Lille
Tél: +33 9 54 28 38 76

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