Boguk Maxim wrote:
Postgres 8.1
Sample test table:
(all queries done on fresh vacuumed analyzed table with statistics on
rub_id and news_dtime set to 1000)
(all table in memory and server do not doing anything other)
media=> \d test_table
Table "public.test_table"
Column | Type | Modifiers
------------+-----------------------------+-----------
id | integer |
rub_id | integer |
news_id | integer |
news_dtime | timestamp without time zone |
Indexes:
"test_table_pk" UNIQUE, btree (id)
"test_table_main_idx" btree (rub_id, news_dtime)
media=> select count(*) from test_table;
count
---------
5834463
media=> select count(distinct rub_id) from test_table;
count
-------
342
Now doing 3 simple query:
First:
media=> EXPLAIN ANALYZE select * from test_table where rub_id IN (5)
order by news_dtime limit 20;
QUERY
PLAN
------------------------------------------------------------------------
------------------------------------------------------------------------
Limit (cost=0.00..10.73 rows=20 width=20) (actual time=0.018..0.121
rows=20 loops=1)
-> Index Scan using test_table_main_idx on test_table
(cost=0.00..29758.11 rows=55447 width=20) (actual time=0.014..0.054
rows=20 loops=1)
Index Cond: (rub_id = 5)
Total runtime: 0.186 ms
Second (almost same but with rub_id 8):
media=> EXPLAIN ANALYZE select * from test_table where rub_id IN (8)
order by news_dtime limit 20;
QUERY
PLAN
------------------------------------------------------------------------
------------------------------------------------------------------------
-
Limit (cost=0.00..1.98 rows=20 width=20) (actual time=0.019..0.121
rows=20 loops=1)
-> Index Scan using test_table_main_idx on test_table
(cost=0.00..45976.37 rows=463684 width=20) (actual time=0.014..0.054
rows=20 loops=1)
Index Cond: (rub_id = 8)
Total runtime: 0.186 ms
Now try with rub_id IN (5,8) (I was assumed query will work 2-10 time
longer max... With almost same plan)
But i'm got bad plan/really slow query:
media=> EXPLAIN ANALYZE select * from test_table where rub_id IN (5,8)
order by news_dtime limit 20;
QUERY PLAN
------------------------------------------------------------------------
------------------------------------------------------------------------
-------------
Limit (cost=103337.45..103337.50 rows=20 width=20) (actual
time=4437.841..4437.976 rows=20 loops=1)
-> Sort (cost=103337.45..104624.26 rows=514725 width=20) (actual
time=4437.836..4437.873 rows=20 loops=1)
Sort Key: news_dtime
-> Bitmap Heap Scan on test_table (cost=3818.96..54506.92
rows=514725 width=20) (actual time=82.139..1100.021 rows=515340 loops=1)
Recheck Cond: ((rub_id = 5) OR (rub_id = 8))
-> BitmapOr (cost=3818.96..3818.96 rows=519131 width=0)
(actual time=80.498..80.498 rows=0 loops=1)
-> Bitmap Index Scan on test_table_main_idx
(cost=0.00..409.06 rows=55447 width=0) (actual time=8.342..8.342
rows=54959 loops=1)
Index Cond: (rub_id = 5)
-> Bitmap Index Scan on test_table_main_idx
(cost=0.00..3409.89 rows=463684 width=0) (actual time=72.146..72.146
rows=460381 loops=1)
Index Cond: (rub_id = 8)
Total runtime: 4458.999 ms
(11 rows)
Ouch.... 25000 slower...
Why planner not try two index scan and merge results...
Try: ORDER BY rub_id, news_dtime
Does that give it enough of a hint?
The problem is you're asking for the 20 oldest regardless of rub_id, so
the index isn't as much use as it might be.
Perhaps an index on (news_dtime,rub_id) rather than the other way around?
--
Richard Huxton
Archonet Ltd
