SQL optimization problem - postgresql

Question

In our product database, ther is a slow sql which runs very frequently , just as the following
csv logs shows that it usually taks more than 1 second , But it is so inconceivable that when I
excute the sql in Database Server, it usually just taks about 0.3 s。From the plan ,we can see
that it uses a right index。 Anybody can explain it and have some optimization advice,thanks !
--sql
SELECT id, content, the_geo, lon, lat, skyid, addtime
FROM skytf.tbl_map
where skytf.ST_Distance_sphere(the_geo, skytf.geometryFromText('POINT(0 -4.0E-5)')) <= 1000
and the_geo && skytf.ST_BUFFER(skytf.geometryfromtext('POINT(0 -4.0E-5)'),0.005)
order by addtime desc limit 30
--csv log
2011-08-30 03:02:06.029 CST,"lbs","skytf",28656,"192.168.168.46:53430",4e5b9d45.6ff0,356,"SELECT",2011-08-29 22:08:05 CST,106/3030952,0,LOG,00000,"duration: 1782.945 ms execute <unnamed>: SELECT id,content,the_geo,lon,lat,skyid,addtime FROM skytf.tbl_map where skytf.ST_Distance_sphere(the_geo,skytf.geometryFromText($1))<=1000 and the_geo && skytf.ST_BUFFER(skytf.geometryfromtext($2),0.005) order by addtime desc limit 30 ","parameters: $1 = 'POINT(0 -4.0E-5)', $2 = 'POINT(0 -4.0E-5)'",,,,,,,,""
There are plenty of the duration logs, I just paste a line 。 ""*duration: 1782.945 ms"*
--explain analyze
skytf=> explain analyze SELECT id,content,the_geo,lon,lat,skyid,addtime FROM skytf.tbl_map
skytf-> where skytf.ST_Distance_sphere(the_geo,skytf.geometryFromText('POINT(0 -4.0E-5)'))<=1000
skytf-> and the_geo && skytf.ST_BUFFER(skytf.geometryfromtext('POINT(0 -4.0E-5)'),0.005)
skytf-> order by addtime desc limit 30 ;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=11118.56..11118.64 rows=30 width=128) (actual time=338.031..338.037 rows=30 loops=1)
-> Sort (cost=11118.56..11124.60 rows=2416 width=128) (actual time=338.030..338.032 rows=30 loops=1)
Sort Key: addtime
Sort Method: top-N heapsort Memory: 29kB
-> Bitmap Heap Scan on tbl_map (cost=201.51..11047.21 rows=2416 width=128) (actual time=53.455..309.962 rows=78121 loops=1)
Recheck Cond: ((the_geo)::box && '(0.005,0.00496),(-0.005,-0.00504)'::box)
Filter: (skytf.st_distance_sphere(the_geo, '01010000000000000000000000F168E388B5F804BF'::skytf.geometry) <=1000::double precision)
-> Bitmap Index Scan on tbl_map_idx_gin (cost=0.00..200.91 rows=7249 width=0) (actual time=49.392..49.392 rows=78559 loops=1)
Index Cond: ((the_geo)::box && '(0.005,0.00496),(-0.005,-0.00504)'::box)
Total runtime: 338.125 ms
(10 rows)
--table information
skytf=> \dt+ skytf.tbl_map
List of relations
Schema | Name | Type | Owner | Size | Description
------------+---------------+-------+-------+--------+-------------
skytf | tbl_map | table | lbs | 158 MB |
(1 row)
skytf=> \d skytf.tbl_map
Table "skytf.tbl_map"
Column | Type | Modifiers
--------------+--------------------------------+-----------------------------------------------------------------------
id | integer | not null default nextval('skytf.tbl_map_id_seq'::regclass)
content | character varying(100) |
lon | double precision |
lat | double precision |
skyid | integer |
addtime | timestamp(1) without time zone | default now()
the_geo | skytf.geometry |
viewcount | integer | default 0
lastreadtime | timestamp without time zone |
ischeck | boolean |
Indexes:
"tbl_map_pkey" PRIMARY KEY, btree (id)
"idx_map_book_skyid" btree (skyid, addtime)
"tbl_map_idx_gin" gist ((the_geo::box))

Answer 1

It is possible to have slower queries while checkpoint is executed.
Try to log checkpoints and if this is your case tune checkpoints for latency. In general this may reduce bytes written per sec. but will improve slow queries.