Bitmap scan when it is not needed

From: Ilia Kantor
Subject: Bitmap scan when it is not needed
Date: ,
Msg-id: auto-000558491990@umail.ru
(view: Whole thread, Raw)
Responses: Re: Bitmap scan when it is not needed  (Tom Lane)
List: pgsql-performance

explain analyze select * from objects_hier where tg && array[0] and id <10000;

 

Bitmap Heap Scan on objects_hier  (cost=4.79..8.80 rows=1 width=337) (actual time=0.110..0.110 rows=0 loops=1)

   Recheck Cond: ((tg && '{0}'::integer[]) AND (id < 10000))

   ->  BitmapAnd  (cost=4.79..4.79 rows=1 width=0) (actual time=0.106..0.106 rows=0 loops=1)

         ->  Bitmap Index Scan on gistbla2  (cost=0.00..2.10 rows=30 width=0) (actual time=0.042..0.042 rows=0 loops=1)

               Index Cond: (tg && '{0}'::integer[])

         ->  Bitmap Index Scan on ohid  (cost=0.00..2.44 rows=1240 width=0) (actual time=0.058..0.058 rows=1255 loops=1)

               Index Cond: (id < 10000)

 

I see, Bitmap is going to AND my indexes.. It read one with less number of rows estimated the first (right)..

It found 0 records at gistbla2 index…

 

Then why is it reading ohid ? 

 

Maybe a quickfix is possible for cases when 0 records is found to stop reading other AND elements..

 


pgsql-performance by date:

From: Tom Lane
Date:
Subject: Re: Bitmap scan when it is not needed
From: Carlos Henrique Reimer
Date:
Subject: shared buffers