bitmap scan and explain analyze
This is a discussion on bitmap scan and explain analyze within the pgsql Hackers forums, part of the PostgreSQL category; --> I noticed that explain analyze emits a little bit funny reslut for bitmap scans: BitmapOr (cost=157.36..157.36 rows=18894 width=0) (actual ...
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04-11-2008, 04:41 AM
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 bitmap scan and explain analyze I noticed that explain analyze emits a little bit funny reslut for bitmap scans: BitmapOr (cost=157.36..157.36 rows=18894 width=0) (actual time=9.406..9.406 rows=0 loops=1) Why actual rows=0? -- Tatsuo Ishii ---------------------------(end of broadcast)--------------------------- TIP 8: explain analyze is your friend      |
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04-11-2008, 04:41 AM
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| Re: bitmap scan and explain analyze Tatsuo Ishii <t-ishii@sra.co.jp> writes: > Why actual rows=0? I couldn't think of any reasonably cheap way to count the actual rows (especially in the presence of lossy bitmaps), so I just punted. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq |
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04-11-2008, 04:44 AM
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| Re: bitmap scan and explain analyze > > Why actual rows=0? > > I couldn't think of any reasonably cheap way to count the actual rows > (especially in the presence of lossy bitmaps), so I just punted. I see. BTW is it possible to let BitmapHeapScan fetch tuples by TID order? It would make heap acccess in sequential manner and would boost disk access performance. -- Tatsuo Ishii ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq |
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04-11-2008, 04:44 AM
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| Re: bitmap scan and explain analyze Tatsuo Ishii <t-ishii@sra.co.jp> writes: > BTW is it possible to let BitmapHeapScan fetch tuples by TID order? It already does ... regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match |
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04-11-2008, 04:44 AM
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| Re: bitmap scan and explain analyze > > BTW is it possible to let BitmapHeapScan fetch tuples by TID order? > > It already does ... Oh great. It seems tbm_begin_iterate() does the trick... -- Tatsuo Ishii ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings |
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