为什么最自然的查询（即使用 INNER JOIN（而不是 LEFT JOIN））非常慢答案

【问题标题】：Why the most natural query(I.e. using INNER JOIN (instead of LEFT JOIN)) is very slow为什么最自然的查询（即使用 INNER JOIN（而不是 LEFT JOIN））非常慢
【发布时间】：2011-03-11 18:15:02
【问题描述】：

此查询耗时过长。

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（请注意 mlist_detail_parameter 和参数表之间多余的 EXISTS），它会使用它的自然查询（即 INNER JOIN）。

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"

尽管这很快，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'))
    )

)

...，何时加入：

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。我可以选择使用不带 LEFT JOIN 的查询，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。

我发现的最佳性能点是只使用 LEFT JOIN，但我认为盲目地应用 LEFT JOIN 可能会导致我进入 Cargo Cult Programming

[编辑：2010-07-06]

该死，我现在是 IN 查询的粉丝，我将 EXISTS 查询转换为 IN 查询，现在速度快得惊人。

这个查询。

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 子句做了一个糟糕的优化，我不认为 EXISTS 子句本质上比 IN 慢。事实上，我记得我之前通过将 IN 子句转换为 EXISTS 子句优化了一个非性能查询（我认为是 8.3 或 8.2）

【问题讨论】：

LEFT JOIN 与 INNER JOIN 的选择与性能无关；这就是域的建模方式。
元评论：有没有办法让它更容易阅读？（他在努力，我知道。）
迈克尔，我不明白“从孩子加入父母”的评论。（除了“内部连接形成笛卡尔积，左侧在右侧查找左侧所有行的数据，右侧查找右侧所有行的左侧数据”之外，我从来没有理解过 JOIN。父子关系在这让我感到困惑。:) 那么，让我们试试这个：你是否试图在七张表中制作笛卡尔积？（这样说起来甚至听起来很昂贵。:) 或者您是否试图从您的第一个表中找到六种关系的信息？
从答案移至 cmets。考虑使用整数类型 id 而不是字符串类型，因为它有惩罚。它没有解释内连接和左外连接之间查询时间的差异，但可能会大大减少整体查询时间。出于某种原因，ID 被转换为文本。我知道这不是灵丹妙药，当然还有很多参数需要考虑。我们有一个使用字符串键的数据提供者，我们使用内部整数键而不是提供的键，它快得多。最大的表包含大约 5 亿行，因此并不庞大。
哇...我现在要上一门关于数据库的课程...看起来可能很有趣：D

标签： sql postgresql optimization

【解决方案1】：

（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。

这在这种情况下效果更好，因为现在将从包含您最大限制因素的查询开始从“由内而外”有效地评估查询：全文搜索查找。该查询将返回一小组行，这些行可以直接针对外部查询的主键（WHERE x in (SELECT X...)）而不是每个值调用一次“内部”查询外部查询（或原始案例中的所有值，如果我没看错的话）。这里的 EXISTS 方法导致嵌套循环（一个查询对另一个查询的每个值的评估）与使用 Hash Joins 的 IN 方法（在许多情况下，如果不是大多数情况下，这是一种更有效的执行方法。

请注意，使用 EXISTS 方法时，有四个嵌套循环执行，每个循环至少运行 3,000 次。这笔费用加起来。虽然不是直接比较，但您可以像对待应用程序代码中的 FOR 循环一样对待嵌套循环：每次调用内部循环时，您的 big-O 估计值都会上升一个数量级：O(n) 到 O(n^ 2) 到 O(n^3) 等。

Hash Join 更像是一张地图，两个数组同时单步执行，并对两个数组进行操作。这大致是线性的 (O(n))。将这些嵌套视为加法，因此它将 O(n) 到 O(2n) 到 O(3n)，等等。

是的，是的，我知道这不是一回事，但关键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 样式更容易识别，我相信.

嵌套循环和 EXISTS 本身并不邪恶，但在大多数情况下，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），IN 表达式（或，在，适当的 JOIN）会产生更有效的计划。

【讨论】：

【解决方案2】：

您的查询本质上是否如下（这很难作为评论提出）：

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，由 Michael Buen 添加]

"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"

【讨论】：

hmm.. 你的答案也很快（0.06 到 0.08 秒之间），但不如 IN 版本快（0.03 到 0.05 秒）。 +1 尽管如此，您的答案仍然有效。您的回答在语义上与我的查询相同。即使，我不能在我的实际代码上使用您的查询，我重构了我的原始查询，以便它可以在两个（或更多）模块上使用，我需要在子查询之外使用 ord_app，我有一些模块可以在上面执行某种 GROUP_CONCAT ord_app。我希望我能 +2 你花时间推断我的代码的意图，并为此编写一个实际的查询:-)
@Michael，我对这个查询的解释分析感兴趣 - 你能编辑这个答案来包含它吗？