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Thread: bitmap scan and explain analyze

bitmap scan and explain analyze

From
Tatsuo Ishii
Date:
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

Re: bitmap scan and explain analyze

From
Tom Lane
Date:
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

Re: bitmap scan and explain analyze

From
Tatsuo Ishii
Date:
> > 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

Re: bitmap scan and explain analyze

From
Tom Lane
Date:
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

Re: bitmap scan and explain analyze

From
Tatsuo Ishii
Date:
> > 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