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They are random as the data are coming
from multiple threads that are inserting in the database. I see
what you say about "linking them", and I may give it a try with
the date. The other think that "links" them together is the 4
georef fields, however at that stage I am trying to collect
statistics on the georefs population of "msg_id" so I don't know
before hand the values to limit my query on them... Do you think
an index on "date, msg_id" might do something?
Yiannis
On 06/08/2012 16:16, David Barton wrote:
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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