Re: different query plan because different limit # (Re: weird query plan)

continue digging shows:
set cpu_tuple_cost to 0.1;
explain analyze select * from wd_urlusermaps where share =1 and
userid='219177' order by id desc limit 20;
SET
时间: 0.256 ms
QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=7063.98..7064.03 rows=20 width=110) (actual
time=6.047..6.130 rows=20 loops=1)
-> Sort (cost=7063.98..7066.71 rows=1094 width=110) (actual
time=6.043..6.070 rows=20 loops=1)
Sort Key: id
-> Index Scan using urlusermaps_userid on wd_urlusermaps
(cost=0.00..7008.76 rows=1094 width=110) (actual time=0.710..5.838
rows=41 loops=1)
Index Cond: (userid = 219177)
Filter: ("share" = 1)
Total runtime: 6.213 ms
(7 rows)

now it's what i need, which means we should increase cpu_tuple_cost for
large
RAM node (we got 16G RAN and the table only serveral hundred M) to avoid
sort
happened too early. is it true?

-laser
> I changed the query to :
> EXPLAIN ANALYZE select id from wd_urlusermaps where id in (select id
> from wd_urlusermaps where share =1 and userid='219177') order by id desc
> limit 20;
>
> and it's much better now (from real execute time), but the cost report
> higher
> then slower one above, may be I should do some tunning on planner
> parameter or
> is it a planner bug?
> QUERY PLAN
>
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=16118.83..16118.88 rows=20 width=4) (actual
> time=17.539..17.619 rows=20 loops=1)
> -> Sort (cost=16118.83..16121.57 rows=1094 width=4) (actual
> time=17.534..17.560 rows=20 loops=1)
> Sort Key: public.wd_urlusermaps.id
> -> Nested Loop (cost=6753.28..16063.61 rows=1094 width=4) (actual
> time=16.739..17.439 rows=41 loops=1)
> -> HashAggregate (cost=6753.28..6764.22 rows=1094 width=4) (actual
> time=16.707..16.786 rows=41 loops=1)
> -> Index Scan using urlusermaps_userid on wd_urlusermaps
> (cost=0.00..6750.55 rows=1094 width=4) (actual time=1.478..16.563
> rows=41 loops=1)
> Index Cond: (userid = 219177)
> Filter: ("share" = 1)
> -> Index Scan using wd_urlusermaps_pkey on wd_urlusermaps
> (cost=0.00..8.49 rows=1 width=4) (actual time=0.008..0.010 rows=1 loops=41)
> Index Cond: (public.wd_urlusermaps.id = public.wd_urlusermaps.id)
> Total runtime: 17.762 ms
> (11 rows)
>
>
>> sorry, forgot to mention our version, it's postgresql 8.2.3
>>
>> -laser
>>
>>
>>> I have a table:
>>> webdigest=# \d wd_urlusermaps
>>> 表 "public.wd_urlusermaps"
>>> 字段名 | 类型 | 修饰词
>>> ---------+-----------------------------+-------------------------------------------------------------
>>> id | integer | not null default nextval('wd_urlusermaps_id_seq'::regclass)
>>> urlid | integer | not null
>>> tag | character varying(512) |
>>> title | character varying(512) |
>>> summary | character varying(1024) |
>>> comment | character varying(1024) |
>>> ctime | timestamp without time zone |
>>> mtime | timestamp without time zone |
>>> share | smallint |
>>> userid | integer |
>>> import | smallint | default 0
>>> 索引:
>>> "wd_urlusermaps_pkey" PRIMARY KEY, btree (id) CLUSTER
>>> "urlusermaps_urlid_userid" UNIQUE, btree (urlid, userid)
>>> "urlusermaps_urlid" btree (urlid)
>>> "urlusermaps_userid" btree (userid)
>>> "wd_urlusermaps_ctime_idx" btree (ctime)
>>> "wd_urlusermaps_share_idx" btree ("share")
>>>
>>> and target statistic set to 1000, and two different query plan:
>>>
>>> webdigest=# explain analyze select A.id as
>>> fav_id,A.urlid,A.tag,A.title,A.summary,A.comment,A.ctime,A.share from
>>> wd_urlusermaps A where share =1 and A.userid='219177' ORDER BY A.id DESC
>>> limit 20 ;
>>> QUERY PLAN
>>>
--------------------------------------------------------------------------------------------------------------------------------------------------------------------
>>> Limit (cost=0.00..4932.56 rows=20 width=96) (actual
>>> time=730.461..2374.435 rows=20 loops=1)
>>> -> Index Scan Backward using wd_urlusermaps_pkey on wd_urlusermaps a
>>> (cost=0.00..269810.77 rows=1094 width=96) (actual time=730.456..2374.367
>>> rows=20 loops=1)
>>> Filter: (("share" = 1) AND (userid = 219177))
>>> Total runtime: 2374.513 ms
>>> (4 rows)
>>>
>>> webdigest=# explain analyze select A.id as
>>> fav_id,A.urlid,A.tag,A.title,A.summary,A.comment,A.ctime,A.share from
>>> wd_urlusermaps A where share =1 and A.userid='219177' ORDER BY A.id DESC
>>> limit 40 ;
>>> QUERY PLAN
>>>
---------------------------------------------------------------------------------------------------------------------------------------------------------
>>> Limit (cost=6805.77..6805.87 rows=40 width=96) (actual time=5.731..5.905
>>> rows=40 loops=1)
>>> -> Sort (cost=6805.77..6808.50 rows=1094 width=96) (actual
>>> time=5.726..5.785 rows=40 loops=1)
>>> Sort Key: id
>>> -> Index Scan using urlusermaps_userid on wd_urlusermaps a
>>> (cost=0.00..6750.55 rows=1094 width=96) (actual time=0.544..5.616
>>> rows=41 loops=1)
>>> Index Cond: (userid = 219177)
>>> Filter: ("share" = 1)
>>> Total runtime: 6.013 ms
>>> (7 rows)
>>>
>>> the userid=219177 got 2000+ record and around 40 shared=1, why above 2 query
>>> shows so much difference?
>>>
>>> any hint would be greatly appreciated.
>>>
>>> -laser
>>>
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