Re: Performance with sorting and LIMIT on partitioned table - Mailing list pgsql-performance

On Mon, Oct 12, 2009 at 10:14 AM, Michal Szymanski <mich20061@gmail.com> wrote: 

We have performance problem with query on partitioned table when query
use order by and we want to use first/last rows from result set.
More detail description:
We have big table where each row is one telephone call (CDR).
Definitnion of this table look like this:
CREATE TABLE accounting.cdr_full_partitioned  (it is parrent table)
(
 cdr_id bigint NOT NULL,
 id_crx_group_from bigint,                                             -- identifier of user
 start_time_invite timestamp with time zone,   -- start call time
 call_status VARCHAR                                                   -- FINF-call finished, FINC-call
unfinished
 ..some extra data..
)

We creating 12 partitions using 'start_time_invite' column, simply we
create one partition for each month. We create costraints like this:
ALTER TABLE accounting.cdr_y2009_m09
 ADD CONSTRAINT y2009m09 CHECK (start_time_invite >= '2009-09-01
00:00:00+02'::timestamp with time zone AND start_time_invite <
'2009-10-01 00:00:00+02'::timestamp with time zone);

and we define necessery indexes of course

CREATE INDEX cdr_full_partitioned_y2009_m09_id_crx_group_to_key1
 ON accounting.cdr_full_partitioned_y2009_m09
 USING btree
 (id_crx_group_from, start_time_invite, call_status);


The problem appears when we want to select calls for specified user
with specified call_Status e.g:
 SELECT * FROM accounting.cdr_full_partitioned
  WHERE
  id_crx_group_from='522921' AND
  call_status='FINS' AND
  start_time_invite>='2009-09-28 00:00:00+02' AND
  start_time_invite<'2009-10-12 23:59:59+02'   AND
 ORDER BY start_time_invite  LIMIT '100' OFFSET 0

you can see execution plan  http://szymanskich.net/pub/postgres/full.jpg
 as you see 20000 rows were selected and after were sorted what take
very long about 30-40s and after sorting it limit
result to 100 rows.

Using table without partition

 SELECT * FROM accounting.cdr_full    WHERE
(id_crx_group_from='522921') AND (
  call_status='FINS' ) AND (start_time_invite>='2009-01-28
00:00:00+02')
  AND (start_time_invite<'2009-10-12 23:59:59+02') ORDER BY
start_time_invite  LIMIT '100' OFFSET 0

execution plan is very simple
"Limit  (cost=0.00..406.40 rows=100 width=456)"
"  ->  Index Scan using
cdr_full_crx_group_from_start_time_invite_status_ind on cdr_full
(cost=0.00..18275.76 rows=4497 width=456)"
"        Index Cond: ((id_crx_group_from = 522921::bigint) AND
(start_time_invite >= '2009-01-27 23:00:00+01'::timestamp with time
zone) AND (start_time_invite < '2009-10-12 23:59:59+02'::timestamp
with time zone) AND ((call_status)::text = 'FINS'::text))"

it use index to fetch first 100 rows and it is super fast and take
less than 0.5s. There is no rows sorting!
I've tried to execute the same query on one partition:
 SELECT * FROM accounting.cdr_full_partitioned_y2009_m09
 WHERE (id_crx_group_from='509498') AND (
  call_status='FINS' ) AND (start_time_invite>='2009-09-01
00:00:00+02')
  AND (start_time_invite<'2009-10-12 23:59:59+02')

You can see execution plan http://szymanskich.net/pub/postgres/ononeprtition.jpg
and query is superfast because there is no sorting. The question is
how to speed up query when we use partitioning? So far I have not
found solution. I'm wonder how do you solve problems
when result from partition must be sorted and after we want to display
only first/last 100 rows?
We can use own partitioning mechanism and partitioning data using
id_crx_group_from and create dynamic query (depending on
id_crx_group_from we can execute query on one partition) but it is not
most beautiful solution.   

Yeah - unfortunately the query planner is not real smart about
partitioned tables yet.  I can't make anything of the JPG link you
posted.  Can you post the EXPLAIN ANALYZE output for the case that is
slow?  What PG version is this?

...Robert
 

I have a similar, recent thread titled Partitioned Tables and ORDER BY with a decent break down.  I think I am hitting the same issue Michal is.

Essentially doing a SELECT against the parent with appropriate constraint columns in the WHERE clause is very fast (uses index scans against correct child table only) but the moment you add an ORDER BY it seems to be merging the parent (an empty table) and the child, sorting the results, and sequential scanning.  So it does still scan only the appropriate child table in the end but indexes are useless.

Unfortunately the only workaround I can come up with is to query the partitioned child tables directly.  In my case the partitions are rather large so the timing difference is 522ms versus 149865ms.