weird query plan

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