Re: Re-Reason of Slowness of Query - Mailing list pgsql-performance

23.03.11 13:21, Adarsh Sharma написав(ла):

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)
pdc_uima-# from page_content p left join clause2 c on (p.crawled_page_id =
pdc_uima(# 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.

Best regards, Vitalii Tymchyshyn