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
