此查询耗时太长。
explain analyze
select
c.company_rec_id,
c.the_company_code ,
c.company
from
tlist t
-- it is questionable why this query become fast when using left join, the most natural query is inner join...
join mlist m using(mlist_rec_id)
join parcel_application ord_app using(parcel_application_rec_id)
join parcel ord using(parcel_rec_id)
join company c on c.company_rec_id = ord.client_rec_id
-- ...questionable
where
(
'cadmium' = ''
or
exists
(
select *
from mlist_detail md
where
md.mlist_rec_id = m.mlist_rec_id
and exists
(
select *
from mlist_detail_parameter mdp
join parameter p using(parameter_rec_id)
where mdp.mlist_detail_rec_id = md.mlist_detail_rec_id
and to_tsvector(extract_words(p.parameter)) @@ plainto_tsquery(extract_words('cadmium'))
)
)
)
group by c.company_rec_id,
c.the_company_code, c.company
......这是执行计划：
"HashAggregate (cost=3791153.43..3791156.38 rows=295 width=71) (actual time=222284.340..222284.357 rows=84 loops=1)"
" -> Nested Loop (cost=0.00..3791139.81 rows=1816 width=71) (actual time=421.412..222282.253 rows=715 loops=1)"
" -> Merge Join (cost=0.00..3790598.03 rows=1816 width=37) (actual time=421.395..222273.876 rows=715 loops=1)"
" Merge Cond: ((ord_app.parcel_rec_id)::text = (ord.parcel_rec_id)::text)"
" -> Nested Loop (cost=0.00..3790229.12 rows=1816 width=37) (actual time=421.371..222264.556 rows=715 loops=1)"
" -> Nested Loop (cost=0.00..3789062.60 rows=1816 width=74) (actual time=421.346..222250.614 rows=715 loops=1)"
" -> Index Scan using fki_parcel_application__parcel on parcel_application ord_app (cost=0.00..508.50 rows=3218 width=74) (actual time=0.019..6.952 rows=3218 loops=1)"
" -> Index Scan using fki_mlist__parcel_application on mlist m (cost=0.00..1177.29 rows=1 width=74) (actual time=66.900..69.060 rows=0 loops=3218)"
" Index Cond: ((m.parcel_application_rec_id)::text = (ord_app.parcel_application_rec_id)::text)"
" Filter: (alternatives: SubPlan 1 or hashed SubPlan 2)"
" SubPlan 1"
" -> Nested Loop Semi Join (cost=3.49..5870.46 rows=5 width=0) (actual time=61.189..61.189 rows=0 loops=3631)"
" Join Filter: ((md.mlist_detail_rec_id)::text = (mdp.mlist_detail_rec_id)::text)"
" -> Index Scan using fki_mlist_detail__mlist on mlist_detail md (cost=0.00..24.31 rows=5 width=37) (actual time=0.014..0.023 rows=4 loops=3631)"
" Index Cond: ((mlist_rec_id)::text = ($0)::text)"
" -> Hash Join (cost=3.49..1163.57 rows=676 width=37) (actual time=0.004..16.045 rows=1865 loops=13473)"
" Hash Cond: ((mdp.parameter_rec_id)::text = (p.parameter_rec_id)::text)"
" -> Seq Scan on mlist_detail_parameter mdp (cost=0.00..1013.87 rows=37187 width=74) (actual time=0.001..5.935 rows=36353 loops=13473)"
" -> Hash (cost=3.48..3.48 rows=1 width=37) (actual time=1.646..1.646 rows=1 loops=1)"
" -> Seq Scan on parameter p (cost=0.00..3.48 rows=1 width=37) (actual time=1.397..1.642 rows=1 loops=1)"
" Filter: (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text))"
" SubPlan 2"
" -> Hash Join (cost=1180.47..3241.32 rows=15198 width=37) (never executed)"
" Hash Cond: ((md.mlist_detail_rec_id)::text = (mdp.mlist_detail_rec_id)::text)"
" -> Seq Scan on mlist_detail md (cost=0.00..1987.79 rows=17679 width=74) (never executed)"
" -> Hash (cost=1172.02..1172.02 rows=676 width=37) (never executed)"
" -> HashAggregate (cost=1165.26..1172.02 rows=676 width=37) (never executed)"
" -> Hash Join (cost=3.49..1163.57 rows=676 width=37) (never executed)"
" Hash Cond: ((mdp.parameter_rec_id)::text = (p.parameter_rec_id)::text)"
" -> Seq Scan on mlist_detail_parameter mdp (cost=0.00..1013.87 rows=37187 width=74) (never executed)"
" -> Hash (cost=3.48..3.48 rows=1 width=37) (never executed)"
" -> Seq Scan on parameter p (cost=0.00..3.48 rows=1 width=37) (never executed)"
" Filter: (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text))"
" -> Index Scan using fki_tlist__mlist on tlist t (cost=0.00..0.63 rows=1 width=37) (actual time=0.015..0.015 rows=1 loops=715)"
" Index Cond: ((t.mlist_rec_id)::text = (m.mlist_rec_id)::text)"
" -> Index Scan using pk_parcel on parcel ord (cost=0.00..338.49 rows=3087 width=74) (actual time=0.008..3.312 rows=3146 loops=1)"
" -> Index Scan using pk_company on company c (cost=0.00..0.29 rows=1 width=71) (actual time=0.009..0.009 rows=1 loops=715)"
" Index Cond: ((c.company_rec_id)::text = (ord.client_rec_id)::text)"
"Total runtime: 222284.584 ms"
当我将其转换为左连接时。
explain analyze
select
c.company_rec_id,
c.the_company_code ,
c.company
from
tlist t
-- it is questionable why this query become fast when using left join, the most natural query is inner join...
left join mlist m using(mlist_rec_id)
left join parcel_application ord_app using(parcel_application_rec_id)
left join parcel ord using(parcel_rec_id)
left join company c on c.company_rec_id = ord.client_rec_id
-- ...questionable
where
(
'cadmium' = ''
or
exists
(
select *
from mlist_detail md
where
md.mlist_rec_id = m.mlist_rec_id
and exists
(
select *
from mlist_detail_parameter mdp
join parameter p using(parameter_rec_id)
where mdp.mlist_detail_rec_id = md.mlist_detail_rec_id
and to_tsvector(extract_words(p.parameter)) @@ plainto_tsquery(extract_words('cadmium'))
)
)
)
group by c.company_rec_id,
c.the_company_code, c.company
......它变得很快：
"HashAggregate (cost=4276494.40..4276497.35 rows=295 width=71) (actual time=71.299..71.309 rows=84 loops=1)"
" -> Nested Loop Left Join (cost=0.00..4276480.78 rows=1816 width=71) (actual time=35.271..70.776 rows=715 loops=1)"
" -> Nested Loop Left Join (cost=0.00..4275939.00 rows=1816 width=37) (actual time=35.260..66.157 rows=715 loops=1)"
" -> Nested Loop Left Join (cost=0.00..4275263.92 rows=1816 width=37) (actual time=35.243..60.185 rows=715 loops=1)"
" -> Merge Left Join (cost=0.00..4274523.56 rows=1816 width=37) (actual time=35.226..54.834 rows=715 loops=1)"
" Merge Cond: ((t.mlist_rec_id)::text = (m.mlist_rec_id)::text)"
" Filter: (alternatives: SubPlan 1 or hashed SubPlan 2)"
" -> Index Scan using fki_tlist__mlist on tlist t (cost=0.00..710.72 rows=3631 width=37) (actual time=0.011..1.947 rows=3631 loops=1)"
" -> Index Scan using pk_mlist on mlist m (cost=0.00..482.71 rows=3631 width=74) (actual time=0.008..1.829 rows=3631 loops=1)"
" SubPlan 1"
" -> Nested Loop Semi Join (cost=3.49..5870.46 rows=5 width=0) (never executed)"
" Join Filter: ((md.mlist_detail_rec_id)::text = (mdp.mlist_detail_rec_id)::text)"
" -> Index Scan using fki_mlist_detail__mlist on mlist_detail md (cost=0.00..24.31 rows=5 width=37) (never executed)"
" Index Cond: ((mlist_rec_id)::text = ($0)::text)"
" -> Hash Join (cost=3.49..1163.57 rows=676 width=37) (never executed)"
" Hash Cond: ((mdp.parameter_rec_id)::text = (p.parameter_rec_id)::text)"
" -> Seq Scan on mlist_detail_parameter mdp (cost=0.00..1013.87 rows=37187 width=74) (never executed)"
" -> Hash (cost=3.48..3.48 rows=1 width=37) (never executed)"
" -> Seq Scan on parameter p (cost=0.00..3.48 rows=1 width=37) (never executed)"
" Filter: (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text))"
" SubPlan 2"
" -> Hash Join (cost=1180.47..3241.32 rows=15198 width=37) (actual time=16.478..34.359 rows=1918 loops=1)"
" Hash Cond: ((md.mlist_detail_rec_id)::text = (mdp.mlist_detail_rec_id)::text)"
" -> Seq Scan on mlist_detail md (cost=0.00..1987.79 rows=17679 width=74) (actual time=0.004..3.013 rows=17679 loops=1)"
" -> Hash (cost=1172.02..1172.02 rows=676 width=37) (actual time=16.461..16.461 rows=1918 loops=1)"
" -> HashAggregate (cost=1165.26..1172.02 rows=676 width=37) (actual time=15.790..16.037 rows=1918 loops=1)"
" -> Hash Join (cost=3.49..1163.57 rows=676 width=37) (actual time=1.613..15.037 rows=1918 loops=1)"
" Hash Cond: ((mdp.parameter_rec_id)::text = (p.parameter_rec_id)::text)"
" -> Seq Scan on mlist_detail_parameter mdp (cost=0.00..1013.87 rows=37187 width=74) (actual time=0.002..4.624 rows=37187 loops=1)"
" -> Hash (cost=3.48..3.48 rows=1 width=37) (actual time=1.593..1.593 rows=1 loops=1)"
" -> Seq Scan on parameter p (cost=0.00..3.48 rows=1 width=37) (actual time=1.311..1.588 rows=1 loops=1)"
" Filter: (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text))"
" -> Index Scan using pk_parcel_application on parcel_application ord_app (cost=0.00..0.40 rows=1 width=74) (actual time=0.007..0.007 rows=1 loops=715)"
" Index Cond: ((m.parcel_application_rec_id)::text = (ord_app.parcel_application_rec_id)::text)"
" -> Index Scan using pk_parcel on parcel ord (cost=0.00..0.36 rows=1 width=74) (actual time=0.007..0.008 rows=1 loops=715)"
" Index Cond: ((ord_app.parcel_rec_id)::text = (ord.parcel_rec_id)::text)"
" -> Index Scan using pk_company on company c (cost=0.00..0.29 rows=1 width=71) (actual time=0.006..0.006 rows=1 loops=715)"
" Index Cond: ((c.company_rec_id)::text = (ord.client_rec_id)::text)"
"Total runtime: 71.504 ms"
如果我这样做，它可以使用它的自然查询（即内部连接）（注意mlist_detail_parameter和参数表之间的多余EXISTS）。
explain analyze
select
c.company_rec_id,
c.the_company_code ,
c.company
from
tlist t
join mlist m using(mlist_rec_id)
join parcel_application ord_app using(parcel_application_rec_id)
join parcel ord using(parcel_rec_id)
join company c on c.company_rec_id = ord.client_rec_id
where
(
'cadmium' = ''
or
exists
(
select *
from mlist_detail md
where
md.mlist_rec_id = m.mlist_rec_id
and exists
(
select * from mlist_detail_parameter mdp
where mdp.mlist_detail_rec_id = md.mlist_detail_rec_id
-- Notice that there is no joining of parameter TO mlist_detail_parameter; We use EXISTS instead, which looks very superfluous
and exists
(
select * from parameter p
where parameter_rec_id = mdp.parameter_rec_id
and to_tsvector(extract_words(p.parameter)) @@ plainto_tsquery(extract_words('cadmium'))
)
)
)
)
group by c.company_rec_id,
c.the_company_code, c.company
......，它有效：
"HashAggregate (cost=76424.17..76427.12 rows=295 width=71) (actual time=398.906..398.932 rows=84 loops=1)"
" -> Nested Loop (cost=0.00..76410.55 rows=1816 width=71) (actual time=52.529..398.171 rows=715 loops=1)"
" -> Merge Join (cost=0.00..75868.77 rows=1816 width=37) (actual time=45.106..385.128 rows=715 loops=1)"
" Merge Cond: ((ord_app.parcel_rec_id)::text = (ord.parcel_rec_id)::text)"
" -> Nested Loop (cost=0.00..75499.86 rows=1816 width=37) (actual time=41.745..374.833 rows=715 loops=1)"
" -> Nested Loop (cost=0.00..74333.34 rows=1816 width=74) (actual time=31.483..302.607 rows=715 loops=1)"
" -> Index Scan using fki_parcel_application__parcel on parcel_application ord_app (cost=0.00..508.50 rows=3218 width=74) (actual time=2.459..5.797 rows=3218 loops=1)"
" -> Index Scan using fki_mlist__parcel_application on mlist m (cost=0.00..22.93 rows=1 width=74) (actual time=0.089..0.092 rows=0 loops=3218)"
" Index Cond: ((m.parcel_application_rec_id)::text = (ord_app.parcel_application_rec_id)::text)"
" Filter: (alternatives: SubPlan 3 or hashed SubPlan 6)"
" SubPlan 3"
" -> Nested Loop Semi Join (cost=0.00..112.64 rows=5 width=0) (actual time=0.071..0.071 rows=0 loops=3631)"
" -> Index Scan using fki_mlist_detail__mlist on mlist_detail md (cost=0.00..24.31 rows=5 width=37) (actual time=0.019..0.023 rows=4 loops=3631)"
" Index Cond: ((mlist_rec_id)::text = ($0)::text)"
" -> Index Scan using fki_mlist_detail_parameter__mlist_detail on mlist_detail_parameter mdp (cost=0.00..19.63 rows=1 width=37) (actual time=0.012..0.012 rows=0 loops=13473)"
" Index Cond: ((mdp.mlist_detail_rec_id)::text = (md.mlist_detail_rec_id)::text)"
" Filter: (alternatives: SubPlan 1 or hashed SubPlan 2)"
" SubPlan 1"
" -> Seq Scan on parameter p (cost=0.00..3.67 rows=1 width=0) (never executed)"
" Filter: (((parameter_rec_id)::text = ($1)::text) AND (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text)))"
" SubPlan 2"
" -> Seq Scan on parameter p (cost=0.00..3.48 rows=1 width=37) (actual time=1.475..1.774 rows=1 loops=1)"
" Filter: (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text))"
" SubPlan 6"
" -> Nested Loop Semi Join (cost=0.00..128665.02 rows=15198 width=37) (never executed)"
" -> Seq Scan on mlist_detail md (cost=0.00..1987.79 rows=17679 width=74) (never executed)"
" -> Index Scan using fki_mlist_detail_parameter__mlist_detail on mlist_detail_parameter mdp (cost=0.00..7.83 rows=1 width=37) (never executed)"
" Index Cond: ((mdp.mlist_detail_rec_id)::text = (md.mlist_detail_rec_id)::text)"
" Filter: (alternatives: SubPlan 4 or hashed SubPlan 5)"
" SubPlan 4"
" -> Seq Scan on parameter p (cost=0.00..3.67 rows=1 width=0) (never executed)"
" Filter: (((parameter_rec_id)::text = ($1)::text) AND (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text)))"
" SubPlan 5"
" -> Seq Scan on parameter p (cost=0.00..3.48 rows=1 width=37) (never executed)"
" Filter: (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text))"
" -> Index Scan using fki_tlist__mlist on tlist t (cost=0.00..0.63 rows=1 width=37) (actual time=0.100..0.100 rows=1 loops=715)"
" Index Cond: ((t.mlist_rec_id)::text = (m.mlist_rec_id)::text)"
" -> Index Scan using pk_parcel on parcel ord (cost=0.00..338.49 rows=3087 width=74) (actual time=3.277..5.920 rows=3146 loops=1)"
" -> Index Scan using pk_company on company c (cost=0.00..0.29 rows=1 width=71) (actual time=0.017..0.017 rows=1 loops=715)"
" Index Cond: ((c.company_rec_id)::text = (ord.client_rec_id)::text)"
"Total runtime: 399.649 ms"
即使这很快，但我认为使用多余的EXISTS是不自然的。
and exists
(
select * from mlist_detail_parameter mdp
where mdp.mlist_detail_rec_id = md.mlist_detail_rec_id
-- Notice that there is no joining of parameter to mlist_detail_parameter, we use EXISTS instead
and exists -- SUPERFLUOUS
(
select * from parameter p
where parameter_rec_id = mdp.parameter_rec_id
and to_tsvector(extract_words(p.parameter)) @@ plainto_tsquery(extract_words('cadmium'))
)
)
...，当加入时会：
and exists
(
select *
from mlist_detail_parameter mdp
join parameter p using(parameter_rec_id)
where mdp.mlist_detail_rec_id = md.mlist_detail_rec_id
and to_tsvector(extract_words(p.parameter)) @@ plainto_tsquery(extract_words('cadmium'))
)
我的问题是为什么最自然的查询（即使用INNER JOIN（而不是LEFT JOIN））非常慢。我可以选择在没有LEFT JOIN的情况下使用查询，但我必须在mlist_detail_parameter和参数表上使用多余的EXISTS，而不是使用直接连接。
我发现最甜蜜的表演点是使用LEFT JOIN，但我认为应用LEFT JOIN 盲目可能会让我进入Cargo Cult Programming
[编辑：2010-07-06]
Darn，我现在是IN查询的粉丝，我将EXISTS查询转换为IN查询，现在速度惊人。
此查询。
explain analyze
select
c.company_rec_id,
c.the_company_code ,
c.company
from
tlist t
-- it is questionable why this query become fast when using left join, the most natural query is inner join...
join mlist m using(mlist_rec_id)
join parcel_application ord_app using(parcel_application_rec_id)
join parcel ord using(parcel_rec_id)
join company c on c.company_rec_id = ord.client_rec_id
-- ...questionable
where
(
'cadmium' = ''
or
mlist_rec_id in
(
select mlist_rec_id
from mlist_detail md
where
mlist_detail_rec_id in
(
select mlist_detail_rec_id
from mlist_detail_parameter mdp
join parameter p using(parameter_rec_id)
where to_tsvector(extract_words(p.parameter)) @@ plainto_tsquery(extract_words('cadmium'))
)
)
)
group by c.company_rec_id,
c.the_company_code, c.company
...有这个执行计划（0.037秒）：
"HashAggregate (cost=4045.22..4048.17 rows=295 width=71) (actual time=37.117..37.128 rows=84 loops=1)"
" -> Hash Join (cost=3734.44..4031.60 rows=1816 width=71) (actual time=33.610..36.650 rows=715 loops=1)"
" Hash Cond: ((ord.client_rec_id)::text = (c.company_rec_id)::text)"
" -> Hash Join (cost=3718.80..3990.99 rows=1816 width=37) (actual time=33.448..36.150 rows=715 loops=1)"
" Hash Cond: ((ord_app.parcel_rec_id)::text = (ord.parcel_rec_id)::text)"
" -> Hash Join (cost=3601.34..3844.02 rows=1816 width=37) (actual time=32.096..34.360 rows=715 loops=1)"
" Hash Cond: ((m.parcel_application_rec_id)::text = (ord_app.parcel_application_rec_id)::text)"
" -> Hash Join (cost=3438.94..3652.11 rows=1816 width=37) (actual time=30.578..32.370 rows=715 loops=1)"
" Hash Cond: ((t.mlist_rec_id)::text = (m.mlist_rec_id)::text)"
" -> Seq Scan on tlist t (cost=3278.24..3459.63 rows=1816 width=37) (actual time=28.930..30.219 rows=715 loops=1)"
" Filter: (hashed SubPlan 1)"
" SubPlan 1"
" -> Hash Join (cost=1179.63..3240.24 rows=15201 width=37) (actual time=17.259..27.282 rows=1918 loops=1)"
" Hash Cond: ((md.mlist_detail_rec_id)::text = (mdp.mlist_detail_rec_id)::text)"
" -> Seq Scan on mlist_detail md (cost=0.00..1987.79 rows=17679 width=74) (actual time=0.002..4.160 rows=17679 loops=1)"
" -> Hash (cost=1171.48..1171.48 rows=652 width=37) (actual time=17.236..17.236 rows=1918 loops=1)"
" -> HashAggregate (cost=1164.96..1171.48 rows=652 width=37) (actual time=16.290..16.692 rows=1918 loops=1)"
" -> Hash Join (cost=3.49..1163.33 rows=652 width=37) (actual time=0.868..15.420 rows=1918 loops=1)"
" Hash Cond: ((mdp.parameter_rec_id)::text = (p.parameter_rec_id)::text)"
" -> Seq Scan on mlist_detail_parameter mdp (cost=0.00..1013.87 rows=37187 width=74) (actual time=0.002..5.397 rows=37187 loops=1)"
" -> Hash (cost=3.48..3.48 rows=1 width=37) (actual time=0.850..0.850 rows=1 loops=1)"
" -> Seq Scan on parameter p (cost=0.00..3.48 rows=1 width=37) (actual time=0.719..0.848 rows=1 loops=1)"
" Filter: (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text))"
" -> Hash (cost=115.31..115.31 rows=3631 width=74) (actual time=1.632..1.632 rows=3631 loops=1)"
" -> Seq Scan on mlist m (cost=0.00..115.31 rows=3631 width=74) (actual time=0.002..0.569 rows=3631 loops=1)"
" -> Hash (cost=122.18..122.18 rows=3218 width=74) (actual time=1.507..1.507 rows=3218 loops=1)"
" -> Seq Scan on parcel_application ord_app (cost=0.00..122.18 rows=3218 width=74) (actual time=0.002..0.531 rows=3218 loops=1)"
" -> Hash (cost=78.87..78.87 rows=3087 width=74) (actual time=1.345..1.345 rows=3087 loops=1)"
" -> Seq Scan on parcel ord (cost=0.00..78.87 rows=3087 width=74) (actual time=0.002..0.469 rows=3087 loops=1)"
" -> Hash (cost=11.95..11.95 rows=295 width=71) (actual time=0.155..0.155 rows=295 loops=1)"
" -> Seq Scan on company c (cost=0.00..11.95 rows=295 width=71) (actual time=0.003..0.063 rows=295 loops=1)"
"Total runtime: 37.240 ms"
一些想法。我认为，最重要的是，Postgresql只是对EXISTS子句进行了一次错误的优化，我不认为EXISTS子句本身比IN慢得多。事实上，我记得我之前通过将IN子句转换为EXISTS子句来优化非信息查询（我认为是8.3或8.2）
答案 0 :(得分：4)
您的查询基本上是以下内容（这很难作为评论）：
select
c.company_rec_id,
c.the_company_code,
c.company
from
company c
where
exists (
select *
from parameter p
join mlist_detail_parameter mdp on mdp.parameter_rec_id = p.parameter_rec_id
join mlist_detail md on md.mlist_detail_rec_id = mdp.mlist_detail_rec_id
join mlist m on m.mlist_rec_id = md.mlist_rec_id
join parcel_application ord_app on ord_app.parcel_application_rec_id = m.parcel_application_rec_id
join parcel ord on ord.parcel_rec_id = ord_app.parcel_rec_id
join tlist t on t.mlist_rec_id = m.mlist_rec_id
where
ord.client_rec_id = c.company_rec_id
and to_tsvector(extract_words(p.parameter)) @@ plainto_tsquery(extract_words('cadmium'))
)
[编辑：2010-07-06，Michael Buen补充]
"Hash Join (cost=2152.94..2172.52 rows=232 width=71) (actual time=71.106..71.207 rows=84 loops=1)"
" Hash Cond: ((c.company_rec_id)::text = (ord.client_rec_id)::text)"
" -> Seq Scan on company c (cost=0.00..11.95 rows=295 width=71) (actual time=0.004..0.030 rows=295 loops=1)"
" -> Hash (cost=2150.04..2150.04 rows=232 width=37) (actual time=71.077..71.077 rows=84 loops=1)"
" -> HashAggregate (cost=2147.72..2150.04 rows=232 width=37) (actual time=71.033..71.040 rows=84 loops=1)"
" -> Nested Loop (cost=1783.22..2146.09 rows=652 width=37) (actual time=51.029..70.187 rows=1918 loops=1)"
" -> Hash Join (cost=1783.22..1938.61 rows=652 width=111) (actual time=51.014..55.913 rows=1918 loops=1)"
" Hash Cond: ((ord_app.parcel_rec_id)::text = (ord.parcel_rec_id)::text)"
" -> Hash Join (cost=1665.76..1810.55 rows=652 width=111) (actual time=48.360..52.004 rows=1918 loops=1)"
" Hash Cond: ((ord_app.parcel_application_rec_id)::text = (m.parcel_application_rec_id)::text)"
" -> Seq Scan on parcel_application ord_app (cost=0.00..122.18 rows=3218 width=74) (actual time=0.003..1.485 rows=3218 loops=1)"
" -> Hash (cost=1657.61..1657.61 rows=652 width=111) (actual time=48.331..48.331 rows=1918 loops=1)"
" -> Hash Join (cost=164.19..1657.61 rows=652 width=111) (actual time=4.755..46.122 rows=1918 loops=1)"
" Hash Cond: ((md.mlist_rec_id)::text = (m.mlist_rec_id)::text)"
" -> Nested Loop (cost=3.49..1485.51 rows=652 width=37) (actual time=1.638..40.974 rows=1918 loops=1)"
" -> Hash Join (cost=3.49..1163.33 rows=652 width=37) (actual time=1.590..18.090 rows=1918 loops=1)"
" Hash Cond: ((mdp.parameter_rec_id)::text = (p.parameter_rec_id)::text)"
" -> Seq Scan on mlist_detail_parameter mdp (cost=0.00..1013.87 rows=37187 width=74) (actual time=0.003..5.499 rows=37187 loops=1)"
" -> Hash (cost=3.48..3.48 rows=1 width=37) (actual time=1.568..1.568 rows=1 loops=1)"
" -> Seq Scan on parameter p (cost=0.00..3.48 rows=1 width=37) (actual time=1.324..1.564 rows=1 loops=1)"
" Filter: (to_tsvector(regexp_replace((parameter)::text, '[\\(\\)\\!\\.\\/,\\-\\?]+'::text, ' '::text, 'g'::text)) @@ plainto_tsquery('cadmium'::text))"
" -> Index Scan using pk_mlist_detail on mlist_detail md (cost=0.00..0.48 rows=1 width=74) (actual time=0.011..0.011 rows=1 loops=1918)"
" Index Cond: ((md.mlist_detail_rec_id)::text = (mdp.mlist_detail_rec_id)::text)"
" -> Hash (cost=115.31..115.31 rows=3631 width=74) (actual time=3.096..3.096 rows=3631 loops=1)"
" -> Seq Scan on mlist m (cost=0.00..115.31 rows=3631 width=74) (actual time=0.003..0.994 rows=3631 loops=1)"
" -> Hash (cost=78.87..78.87 rows=3087 width=74) (actual time=2.640..2.640 rows=3087 loops=1)"
" -> Seq Scan on parcel ord (cost=0.00..78.87 rows=3087 width=74) (actual time=0.004..0.876 rows=3087 loops=1)"
" -> Index Scan using fki_tlist__mlist on tlist t (cost=0.00..0.31 rows=1 width=37) (actual time=0.006..0.006 rows=1 loops=1918)"
" Index Cond: ((t.mlist_rec_id)::text = (m.mlist_rec_id)::text)"
"Total runtime: 71.373 ms"
答案 1 :(得分：4)
（按照指示，我将部分评论放在答案中，因为它解决了问题）
将EXISTS表达式转换为IN表达式。
在这种情况下，这样做效果更好，因为现在可以从包含最大限制因素的查询“内向外”有效地评估查询：全文搜索查找。该查询将返回一小组行，可以直接查询外部查询的主键（WHERE x in（SELECT X ...）），而不是每个值调用一次“内部”查询外部查询（或者对于原始案例中的所有值，如果我正确读取它）。这里的EXISTS方法导致嵌套循环（对另一个值中的每个值的一个查询的一个评估）与使用散列连接的IN方法（在许多（如果不是大多数情况下）情况下更有效的执行方法。
请注意，使用EXISTS方法，有四个嵌套循环执行，每个循环运行至少3,000次。这个成本加起来。虽然它不是直接比较，但您可以像处理应用程序代码中的FOR循环一样处理嵌套循环：每次调用内循环时，您的big-O估计值会上升一个数量级：O（n）到O（n ^ 2）到O（n ^ 3）等。
散列连接更像是一个映射，其中两个数组同时逐步执行，并对两者执行操作。这大致是线性的（O（n））。可以认为它们被嵌套为添加剂，因此它会从O（n）到O（2n）到O（3n）等。
是的，是的，我知道这不是一回事，但重点是拥有多个嵌套循环通常表示查询计划缓慢，并且比较两个大O风格使其更容易识别，我相信的
嵌套循环和EXISTS本身并不是邪恶的，但是对于大多数情况，如果存在最终影响所有内容的基本过滤条件（例如，问题中的全文搜索），则IN表达式（或，在< em>一些案例，一个正确的JOIN）会产生一个更有效的计划。