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Re: Sequential scan instead of index scan

From: David Barton <dave(at)oneit(dot)com(dot)au>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Sequential scan instead of index scan
Date: 2012-08-06 15:16:29
Message-ID: 501FDFCD.2030806@oneit.com.au (view raw or flat)
Thread:
Lists: pgsql-performance
Hi Yiannis,

Is there anything linking these ids together, or are the relatively 
random?  If they are relatively random, the rows are likely to be 
sprinkled amongst many blocks and so a seq scan is the fastest. I've 
seen similar problems with indexed queries in a multi-tennant database 
where the data is so fragmented that once the record volume hits a 
certain threshold, Postgres decides to table scan rather than use an index.

The query optimiser is unlikely to be able to determine the disk 
locality of 300k rows and so it just takes a punt on a seq scan.

If you added another filter condition on something indexed e.g. last 
week or last month or location or something, you might do better if the 
data does exhibit disk locality.  If the data really is scattered, then 
a seq scan really will be quicker.

Regards, David

On 06/08/12 23:08, Ioannis Anagnostopoulos wrote:
> Hi, my query is very simple:
>
> select
>             msg_id,
>             msg_type,
>             ship_pos_messages.pos_georef1,
>             ship_pos_messages.pos_georef2,
>             ship_pos_messages.pos_georef3,
>             ship_pos_messages.pos_georef4,
>             obj_id,
>             ship_speed,
>             ship_heading,
>             ship_course,
>             pos_point
>         from
>             feed_all_y2012m08.ship_pos_messages
>         where
>             extract('day' from msg_date_rec) = 1
>             AND msg_id = any(ARRAY[7294724,14174174,22254408]);
>
> The msg_id is the pkey on the ship_pos_messages table and in this 
> example it is working fast as it uses the pkey (primary key index) to 
> make the selection. The expplain anayze follows:
> "Result  (cost=0.00..86.16 rows=5 width=117) (actual 
> time=128.734..163.319 rows=3 loops=1)"
> "  ->  Append  (cost=0.00..86.16 rows=5 width=117) (actual 
> time=128.732..163.315 rows=3 loops=1)"
> "        ->  Seq Scan on ship_pos_messages  (cost=0.00..0.00 rows=1 
> width=100) (actual time=0.001..0.001 rows=0 loops=1)"
> "              Filter: ((msg_id = ANY 
> ('{7294724,14174174,22254408}'::integer[])) AND 
> (date_part('day'::text, msg_date_rec) = 1::double precision))"
> "        ->  Seq Scan on ship_a_pos_messages ship_pos_messages  
> (cost=0.00..0.00 rows=1 width=100) (actual time=0.000..0.000 rows=0 
> loops=1)"
> "              Filter: ((msg_id = ANY 
> ('{7294724,14174174,22254408}'::integer[])) AND 
> (date_part('day'::text, msg_date_rec) = 1::double precision))"
> "        ->  Bitmap Heap Scan on ship_b_std_pos_messages 
> ship_pos_messages  (cost=13.41..25.42 rows=1 width=128) (actual 
> time=49.127..49.127 rows=0 loops=1)"
> "              Recheck Cond: (msg_id = ANY 
> ('{7294724,14174174,22254408}'::integer[]))"
> "              Filter: (date_part('day'::text, msg_date_rec) = 
> 1::double precision)"
> "              ->  Bitmap Index Scan on ship_b_std_pos_messages_pkey  
> (cost=0.00..13.41 rows=3 width=0) (actual time=49.125..49.125 rows=0 
> loops=1)"
> "                    Index Cond: (msg_id = ANY 
> ('{7294724,14174174,22254408}'::integer[]))"
> "        ->  Bitmap Heap Scan on ship_b_ext_pos_messages 
> ship_pos_messages  (cost=12.80..24.62 rows=1 width=128) (actual 
> time=0.029..0.029 rows=0 loops=1)"
> "              Recheck Cond: (msg_id = ANY 
> ('{7294724,14174174,22254408}'::integer[]))"
> "              Filter: (date_part('day'::text, msg_date_rec) = 
> 1::double precision)"
> "              ->  Bitmap Index Scan on ship_b_ext_pos_messages_pkey  
> (cost=0.00..12.80 rows=3 width=0) (actual time=0.027..0.027 rows=0 
> loops=1)"
> "                    Index Cond: (msg_id = ANY 
> ('{7294724,14174174,22254408}'::integer[]))"
> "        ->  Bitmap Heap Scan on ship_a_pos_messages_wk0 
> ship_pos_messages  (cost=24.08..36.12 rows=1 width=128) (actual 
> time=79.572..114.152 rows=3 loops=1)"
> "              Recheck Cond: (msg_id = ANY 
> ('{7294724,14174174,22254408}'::integer[]))"
> "              Filter: (date_part('day'::text, msg_date_rec) = 
> 1::double precision)"
> "              ->  Bitmap Index Scan on ship_a_pos_messages_wk0_pkey  
> (cost=0.00..24.08 rows=3 width=0) (actual time=67.441..67.441 rows=3 
> loops=1)"
> "                    Index Cond: (msg_id = ANY 
> ('{7294724,14174174,22254408}'::integer[]))"
> "Total runtime: 180.146 ms"
>
> I think this is a pretty good plan and quite quick given the size of 
> the table (88Million rows at present). However in real life the 
> parameter where I search for msg_id is not an array of 3 ids but of 
> 300.000 or more. It is then that the query forgets the plan and goes 
> to sequential scan. Is there any way around? Or is this the best I can 
> have?
>
> Kind Regards
> Yiannis

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