Re: Re-Reason of Slowness of Query

Vitalii Tymchyshyn wrote:
> 23.03.11 13:21, Adarsh Sharma ???????(??):
>> Thank U all, for U'r Nice Support.
>>
>> Let me Conclude the results, below results are obtained after finding
>> the needed queries :
>>
>> *First Option :
>>
>> *pdc_uima=# explain analyze select distinct(p.crawled_page_id) from
>> page_content p left join clause2 c on (p.crawled_page_id =
>> c.source_id) where (c.source_id is null);
>>
>> QUERY
>> PLAN
>>
>> -----------------------------------------------------------------------------------------------------------------------------------------------------
>> HashAggregate (cost=100278.16..104104.75 rows=382659 width=8)
>> (actual time=87927.000..87930.084 rows=72 loops=1)
>> -> Nested Loop Anti Join (cost=0.00..99320.46 rows=383079
>> width=8) (actual time=0.191..87926.546 rows=74 loops=1)
>> -> Seq Scan on page_content p (cost=0.00..87132.17
>> rows=428817 width=8) (actual time=0.027..528.978 rows=428467 loops=1)
>> -> Index Scan using idx_clause2_source_id on clause2 c
>> (cost=0.00..18.18 rows=781 width=4) (actual time=0.202..0.202 rows=1
>> loops=428467)
>> Index Cond: (p.crawled_page_id = c.source_id)
>> Total runtime: 87933.882 ms :-(
>> (6 rows)
>>
>> *Second Option :
>>
>> *pdc_uima=# explain analyze select distinct(p.crawled_page_id) from
>> page_content p
>> pdc_uima-# where NOT EXISTS (select 1 from clause2 c where
>> c.source_id = p.crawled_page_id);
>>
>> QUERY
>> PLAN
>>
>> -----------------------------------------------------------------------------------------------------------------------------------------------------
>> HashAggregate (cost=100278.16..104104.75 rows=382659 width=8)
>> (actual time=7047.259..7050.261 rows=72 loops=1)
>> -> Nested Loop Anti Join (cost=0.00..99320.46 rows=383079
>> width=8) (actual time=0.039..7046.826 rows=74 loops=1)
>> -> Seq Scan on page_content p (cost=0.00..87132.17
>> rows=428817 width=8) (actual time=0.008..388.976 rows=428467 loops=1)
>> -> Index Scan using idx_clause2_source_id on clause2 c
>> (cost=0.00..18.18 rows=781 width=4) (actual time=0.013..0.013 rows=1
>> loops=428467)
>> Index Cond: (c.source_id = p.crawled_page_id)
>> Total runtime: 7054.074 ms :-)
>> (6 rows)
>>
>
> Actually the plans are equal, so I suppose it depends on what were run
> first :). Slow query operates with data mostly on disk, while fast one
> with data in memory.

Yes U 'r absolutely right, if I run it again, it display the output as :

pdc_uima=# explain analyze select distinct(p.crawled_page_id) from
page_content p left join clause2 c on (p.crawled_page_id = c.source_id)
where (c.source_id is null);

QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=100278.16..104104.75 rows=382659 width=8) (actual
time=7618.452..7621.427 rows=72 loops=1)
-> Nested Loop Anti Join (cost=0.00..99320.46 rows=383079 width=8)
(actual time=0.131..7618.043 rows=74 loops=1)
-> Seq Scan on page_content p (cost=0.00..87132.17
rows=428817 width=8) (actual time=0.020..472.811 rows=428467 loops=1)
-> Index Scan using idx_clause2_source_id on clause2 c
(cost=0.00..18.18 rows=781 width=4) (actual time=0.015..0.015 rows=1
loops=428467)
Index Cond: (p.crawled_page_id = c.source_id)
Total runtime: 7637.132 ms
(6 rows)

I let U know after a fresh start (session ).
Then the true result comes and if further tuning required can be performed.

Best Regards, Adarsh
>
> Best regards, Vitalii Tymchyshyn