Обсуждение: Indexing problem with OFFSET LIMIT
I have problem in my applications and don't know how to fix it.
This is the table and one of the indexes:
CREATE TABLE foo
(
id serial NOT NULL,
foo_name character varying(100),
realm_id integer
... and about 50 other columns
)
CREATE INDEX idx_foo_name_realm
ON foo
USING btree
(realm_id, foo_name);
Table foo contains about 8 Million Rows.
The problem:
Consider this query:
SELECT * FROM foo WHERE realm_id = 228 order by foo_name LIMIT 200 OFFSET 15000
And it's execution plan:
"Limit (cost=57527.13..58294.16 rows=200 width=575) (actual time=182.302..184.971 rows=200 loops=1)"
" -> Index Scan using idx_foo_name_realm on foo (cost=0.00..62159.98 rows=16208 width=575) (actual time=0.085..166.861 rows=15200 loops=1)"
" Index Cond: (realm_id = 228)"
"Total runtime: 185.591 ms"
And now look at this:
SELECT * FROM foo WHERE realm_id = 228 order by foo_name LIMIT 200 OFFSET 15999
"Limit (cost=59601.92..59602.42 rows=200 width=575) (actual time=1069.759..1072.310 rows=200 loops=1)"
" -> Sort (cost=59561.92..59602.44 rows=16208 width=575) (actual time=929.948..1052.620 rows=16199 loops=1)"
" Sort Key: foo_name"
" Sort Method: external merge Disk: 8984kB"
" -> Bitmap Heap Scan on foo (cost=306.69..54270.62 rows=16208 width=575) (actual time=9.612..235.902 rows=21788 loops=1)"
" Recheck Cond: (realm_id = 228)"
" -> Bitmap Index Scan on foo_realm_id (cost=0.00..302.64 rows=16208 width=0) (actual time=8.733..8.733 rows=21810 loops=1)"
" Index Cond: (realm_id = 228)"
"Total runtime: 1084.706 ms"
Execution time increases tenfold because postgres stopped using the index.
Can anybody explain to me what's going on and what can be done? Is this a memory problem?
I’m no expert at reading query plans, but I’m guessing the planner chose the other plan because your offset + limit went beyond the row estimate.
Look’s like it’s then doing a disk based sort in the other plan which probably explain why it’s slow.
Someone please correct me if I’m wrong.
Perhaps if you can spare a little more work_mem in postgesql.conf it might go back to a memory sort.
David.
-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Oliver Weichhold
Sent: 29 August 2008 21:38
To: pgsql-general@postgresql.org
Subject: [GENERAL] Indexing problem with OFFSET LIMIT
Hello
I have problem in my applications and don't know how to fix it.
This is the table and one of the indexes:
CREATE TABLE foo
(
id serial NOT NULL,
foo_name character varying(100),
realm_id integer
... and about 50 other columns
)
CREATE INDEX idx_foo_name_realm
ON foo
USING btree
(realm_id, foo_name);
Table foo contains about 8 Million Rows.
The problem:
Consider this query:
SELECT * FROM foo WHERE realm_id = 228 order by foo_name LIMIT 200 OFFSET 15000
And it's execution plan:
"Limit (cost=57527.13..58294.16 rows=200 width=575) (actual time=182.302..184.971 rows=200 loops=1)"
" -> Index Scan using idx_foo_name_realm on foo (cost=0.00..62159.98 rows=16208 width=575) (actual time=0.085..166.861 rows=15200 loops=1)"
" Index Cond: (realm_id = 228)"
"Total runtime: 185.591 ms"
And now look at this:
SELECT * FROM foo WHERE realm_id = 228 order by foo_name LIMIT 200 OFFSET 15999
"Limit (cost=59601.92..59602.42 rows=200 width=575) (actual time=1069.759..1072.310 rows=200 loops=1)"
" -> Sort (cost=59561.92..59602.44 rows=16208 width=575) (actual time=929.948..1052.620 rows=16199 loops=1)"
" Sort Key: foo_name"
" Sort Method: external merge Disk: 8984kB"
" -> Bitmap Heap Scan on foo (cost=306.69..54270.62 rows=16208 width=575) (actual time=9.612..235.902 rows=21788 loops=1)"
" Recheck Cond: (realm_id = 228)"
" -> Bitmap Index Scan on foo_realm_id (cost=0.00..302.64 rows=16208 width=0) (actual time=8.733..8.733 rows=21810 loops=1)"
" Index Cond: (realm_id = 228)"
"Total runtime: 1084.706 ms"
Execution time increases tenfold because postgres stopped using the index.
Can anybody explain to me what's going on and what can be done? Is this a memory problem?
On Fri, Aug 29, 2008 at 4:38 PM, Oliver Weichhold <oliver@weichhold.com> wrote: > Hello > > I have problem in my applications and don't know how to fix it. > > This is the table and one of the indexes: > > CREATE TABLE foo > ( > id serial NOT NULL, > foo_name character varying(100), > realm_id integer > > ... and about 50 other columns > ) > > CREATE INDEX idx_foo_name_realm > ON foo > USING btree > (realm_id, foo_name); > > Table foo contains about 8 Million Rows. > > > The problem: > > Consider this query: > > SELECT * FROM foo WHERE realm_id = 228 order by foo_name LIMIT 200 OFFSET > 15000 try this: SELECT * FROM foo WHERE realm_id = 228 order by realm_id, foo_name LIMIT 200 OFFSET 15000 Or even better don't use 'offset' at all. It's simply lousy. If you want to skip ahead 200 rows at a time, save off the previous last extracted rows in the app: 1st time: select * from foo order by realm_id, foo_name limit 200; times after that: select * from foo where (realm_id, foo_name) > (last_realm_id, last_foo_name) order by realm_id, foo_name limit 200; you should be pleasantly surprised :-). This is also a little bit more graceful if other sessions are deleting/inserting rows while you are browsing. merlin
"Merlin Moncure" <mmoncure@gmail.com> writes:
> On Fri, Aug 29, 2008 at 4:38 PM, Oliver Weichhold <oliver@weichhold.com> wrote:
>> Consider this query:
>>
>> SELECT * FROM foo WHERE realm_id = 228 order by foo_name LIMIT 200 OFFSET
>> 15000
> try this:
> SELECT * FROM foo WHERE realm_id = 228 order by realm_id, foo_name
> LIMIT 200 OFFSET
> 15000
> Or even better don't use 'offset' at all. It's simply lousy. If
> you want to skip ahead 200 rows at a time, save off the previous last
> extracted rows in the app:
Yeah, large OFFSET values are always going to suck performance-wise,
because there is no magic way to skip over those rows --- the backend
has to read them, and then throw them away internally. Avoid OFFSET.
Aside from the type of trick Merlin mentions, have you considered using
a cursor?
regards, tom lane