本節介紹了PostgreSQL中的“大表”連線，這裡的大表是相對的概念，是指表資料量相對於主機資源很大。

**資料庫配置**
主機CPU 4核，記憶體4G，PG共享快取128MB，work_mem 4MB。

**測試資料**
建立4張表，每張表1000w行，資料量約1G，是PG共享記憶體的8倍。
```
drop table t_big_1;
drop table t_big_2;
drop table t_big_3;
drop table t_big_4;

create table t_big_1(id int,c1 varchar(30),c2 varchar(30),c3 varchar(30));
create table t_big_2(id int,c1 varchar(30),c2 varchar(30),c3 varchar(30));
create table t_big_3(id int,c1 varchar(30),c2 varchar(30),c3 varchar(30));
create table t_big_4(id int,c1 varchar(30),c2 varchar(30),c3 varchar(30));

insert into t_big_1 select x,rpad('c1'||x,30,'c1'),rpad('c2'||x,30,'c2'),rpad('c3'||x,30,'c3') from generate_series(1,10000000) as x;
insert into t_big_2 select x,rpad('c1'||x,30,'c1'),rpad('c2'||x,30,'c2'),rpad('c3'||x,30,'c3') from generate_series(1,10000000) as x;
insert into t_big_3 select x,rpad('c1'||x,30,'c1'),rpad('c2'||x,30,'c2'),rpad('c3'||x,30,'c3') from generate_series(1,10000000) as x;
insert into t_big_4 select x,rpad('c1'||x,30,'c1'),rpad('c2'||x,30,'c2'),rpad('c3'||x,30,'c3') from generate_series(1,10000000) as x;

show shared_buffers;
show effective_cache_size;
show work_mem;

select pg_size_pretty(pg_table_size('t_big_1'));
select pg_size_pretty(pg_table_size('t_big_2'));
select pg_size_pretty(pg_table_size('t_big_3'));
select pg_size_pretty(pg_table_size('t_big_4'));

analyze t_big_1,t_big_2,t_big_3,t_big_4;

explain verbose
select a.*
from t_big_1 a join t_big_2 b on a.c1 = b.c1;

explain verbose
select a.id,b.c1,c.c2,d.c3 
from t_big_1 a,t_big_2 b,t_big_3 c,t_big_4 d
where a.id = b.id and b.id = c.id and c.id = d.id;

explain verbose
select a.id,b.c1,c.c2,d.c3 
from t_big_1 a,t_big_2 b,t_big_3 c,t_big_4 d
where a.id = b.id and b.c1 = c.c1 and c.c2 = d.c2;

```

**大表連線**
未分析資料表前
```
[local:/data/run/pg12]:5120 pg12@testdb=# explain verbose
pg12@testdb-# select a.id,b.c1,c.c2,d.c3 
pg12@testdb-# from t_big_1 a,t_big_2 b,t_big_3 c,t_big_4 d
pg12@testdb-# where a.id = b.id and b.c1 = c.c1 and c.c2 = d.c2;
                                                   QUERY PLAN                                                    
-----------------------------------------------------------------------------------------------------------------
 Merge Join  (cost=164722831406.26..1096915306139605248.00 rows=73127676034285903872 width=238)
   Output: a.id, b.c1, c.c2, d.c3
   Merge Cond: ((b.c1)::text = (c.c1)::text)
   ->  Sort  (cost=58799667920.13..59102008117.66 rows=120936079012 width=82)
         Output: a.id, b.c1
         Sort Key: b.c1
         ->  Merge Join  (cost=2124653.55..1816202724.10 rows=120936079012 width=82)
               Output: a.id, b.c1
               Merge Cond: (a.id = b.id)
               ->  Sort  (cost=894232.27..906527.40 rows=4918050 width=4)
                     Output: a.id
                     Sort Key: a.id
                     ->  Seq Scan on public.t_big_1 a  (cost=0.00..213115.50 rows=4918050 width=4)
                           Output: a.id
               ->  Materialize  (cost=1230421.27..1255011.52 rows=4918050 width=82)
                     Output: b.c1, b.id
                     ->  Sort  (cost=1230421.27..1242716.40 rows=4918050 width=82)
                           Output: b.c1, b.id
                           Sort Key: b.id
                           ->  Seq Scan on public.t_big_2 b  (cost=0.00..213115.50 rows=4918050 width=82)
                                 Output: b.c1, b.id
   ->  Materialize  (cost=105923163486.13..106527843881.19 rows=120936079012 width=234)
         Output: c.c2, c.c1, d.c3
         ->  Sort  (cost=105923163486.13..106225503683.66 rows=120936079012 width=234)
               Output: c.c2, c.c1, d.c3
               Sort Key: c.c1
               ->  Merge Join  (cost=3066006.55..1817144077.10 rows=120936079012 width=234)
                     Output: c.c2, c.c1, d.c3
                     Merge Cond: ((c.c2)::text = (d.c2)::text)
                     ->  Sort  (cost=1533003.27..1545298.40 rows=4918050 width=156)
                           Output: c.c2, c.c1
                           Sort Key: c.c2
                           ->  Seq Scan on public.t_big_3 c  (cost=0.00..213115.50 rows=4918050 width=156)
                                 Output: c.c2, c.c1
                     ->  Materialize  (cost=1533003.27..1557593.52 rows=4918050 width=156)
                           Output: d.c3, d.c2
                           ->  Sort  (cost=1533003.27..1545298.40 rows=4918050 width=156)
                                 Output: d.c3, d.c2
                                 Sort Key: d.c2
                                 ->  Seq Scan on public.t_big_4 d  (cost=0.00..213115.50 rows=4918050 width=156)
                                       Output: d.c3, d.c2
(41 rows)

```
可以看到，未分析前，執行計劃使用merge join，計劃的cost是一個大數。

執行分析後
```

[local:/data/run/pg12]:5120 pg12@testdb=# explain (analyze,buffers,verbose)
select a.id,b.c1,c.c2,d.c3 
from t_big_1 a,t_big_2 b,t_big_3 c,t_big_4 d
where a.id = b.id and b.c1 = c.c1 and c.c2 = d.c2;
                                                                              QUERY PLAN                                                                               
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Gather  (cost=896126.19..2564935.91 rows=9999844 width=97) (actual time=393803.655..404902.025 rows=10000000 loops=1)
   Output: a.id, b.c1, c.c2, d.c3
   Workers Planned: 2
   Workers Launched: 2
   Buffers: shared hit=266 read=655676 dirtied=486717 written=486414, temp read=473954 written=486068
   ->  Parallel Hash Join  (cost=895126.19..1563951.51 rows=4166602 width=97) (actual time=393672.896..398825.027 rows=3333333 loops=3)
         Output: a.id, b.c1, c.c2, d.c3
         Hash Cond: ((c.c2)::text = (d.c2)::text)
         Buffers: shared hit=266 read=655676 dirtied=486717 written=486414, temp read=473954 written=486068
         Worker 0: actual time=393629.565..399028.498 rows=3549817 loops=1
           Buffers: shared hit=118 read=218079 dirtied=161599 written=161495, temp read=162307 written=161880
         Worker 1: actual time=393585.994..399049.295 rows=3609509 loops=1
           Buffers: shared hit=119 read=217313 dirtied=161014 written=160913, temp read=163324 written=160736
         ->  Parallel Hash Join  (cost=592683.65..1070481.02 rows=4166681 width=66) (actual time=328335.871..378143.916 rows=3333333 loops=3)
               Output: a.id, b.c1, c.c2
               Hash Cond: ((b.c1)::text = (c.c1)::text)
               Buffers: shared hit=63 read=491773 dirtied=352782 written=352575, temp read=267125 written=274312
               Worker 0: actual time=328475.430..378240.528 rows=3325497 loops=1
                 Buffers: shared hit=25 read=164024 dirtied=117445 written=117373, temp read=88941 written=91448
               Worker 1: actual time=328084.038..377943.176 rows=3311112 loops=1
                 Buffers: shared hit=29 read=163900 dirtied=117550 written=117481, temp read=88747 written=91320
               ->  Parallel Hash Join  (cost=290238.33..609558.42 rows=4166681 width=35) (actual time=158380.042..198763.345 rows=3333333 loops=3)
                     Output: a.id, b.c1
                     Hash Cond: (a.id = b.id)
                     Buffers: shared hit=63 read=327838 dirtied=218847 written=218710, temp read=98317 written=100856
                     Worker 0: actual time=158518.764..199077.411 rows=3331104 loops=1
                       Buffers: shared hit=25 read=109394 dirtied=72893 written=72845, temp read=32790 written=33668
                     Worker 1: actual time=158520.409..198920.394 rows=3332824 loops=1
                       Buffers: shared hit=29 read=109323 dirtied=73002 written=72956, temp read=32934 written=33560
                     ->  Parallel Seq Scan on public.t_big_1 a  (cost=0.00..205601.81 rows=4166681 width=4) (actual time=239.830..75704.152 rows=3333333 loops=3)
                           Output: a.id
                           Buffers: shared read=163935 dirtied=109449 written=109391
                           Worker 0: actual time=239.584..75677.703 rows=3327794 loops=1
                             Buffers: shared read=54554 dirtied=36489 written=36468
                           Worker 1: actual time=240.355..75258.837 rows=3347802 loops=1
                             Buffers: shared read=54882 dirtied=36486 written=36467
                     ->  Parallel Hash  (cost=205601.81..205601.81 rows=4166681 width=35) (actual time=65812.428..65812.431 rows=3333333 loops=3)
                           Output: b.c1, b.id
                           Buckets: 65536  Batches: 256  Memory Usage: 3328kB
                           Buffers: shared hit=32 read=163903 dirtied=109398 written=109319, temp written=70136
                           Worker 0: actual time=65812.900..65812.904 rows=3345876 loops=1
                             Buffers: shared hit=11 read=54840 dirtied=36404 written=36377, temp written=23428
                           Worker 1: actual time=65812.873..65812.875 rows=3321816 loops=1
                             Buffers: shared hit=15 read=54441 dirtied=36516 written=36489, temp written=23320
                           ->  Parallel Seq Scan on public.t_big_2 b  (cost=0.00..205601.81 rows=4166681 width=35) (actual time=1.490..47839.237 rows=3333333 loops=3)
                                 Output: b.c1, b.id
                                 Buffers: shared hit=32 read=163903 dirtied=109398 written=109319
                                 Worker 0: actual time=1.464..47814.446 rows=3345876 loops=1
                                   Buffers: shared hit=11 read=54840 dirtied=36404 written=36377
                                 Worker 1: actual time=1.470..47104.413 rows=3321816 loops=1
                                   Buffers: shared hit=15 read=54441 dirtied=36516 written=36489
               ->  Parallel Hash  (cost=205601.81..205601.81 rows=4166681 width=62) (actual time=113720.080..113720.080 rows=3333333 loops=3)
                     Output: c.c2, c.c1
                     Buckets: 65536  Batches: 512  Memory Usage: 2432kB
                     Buffers: shared read=163935 dirtied=133935 written=133865, temp written=103856
                     Worker 0: actual time=113719.124..113719.124 rows=3332395 loops=1
                       Buffers: shared read=54630 dirtied=44552 written=44528, temp written=34648
                     Worker 1: actual time=113720.557..113720.558 rows=3329197 loops=1
                       Buffers: shared read=54577 dirtied=44548 written=44525, temp written=34576
                     ->  Parallel Seq Scan on public.t_big_3 c  (cost=0.00..205601.81 rows=4166681 width=62) (actual time=0.126..80608.068 rows=3333333 loops=3)
                           Output: c.c2, c.c1
                           Buffers: shared read=163935 dirtied=133935 written=133865
                           Worker 0: actual time=0.260..80737.065 rows=3332395 loops=1
                             Buffers: shared read=54630 dirtied=44552 written=44528
                           Worker 1: actual time=0.049..80943.448 rows=3329197 loops=1
                             Buffers: shared read=54577 dirtied=44548 written=44525
         ->  Parallel Hash  (cost=205601.02..205601.02 rows=4166602 width=62) (actual time=10279.722..10279.722 rows=3333333 loops=3)
               Output: d.c3, d.c2
               Buckets: 65536  Batches: 512  Memory Usage: 2400kB
               Buffers: shared hit=32 read=163903 dirtied=133935 written=133839, temp written=103004
               Worker 0: actual time=10222.812..10222.812 rows=3297904 loops=1
                 Buffers: shared hit=9 read=54055 dirtied=44154 written=44122, temp written=34236
               Worker 1: actual time=10222.839..10222.839 rows=3258559 loops=1
                 Buffers: shared hit=6 read=53413 dirtied=43464 written=43432, temp written=33504
               ->  Parallel Seq Scan on public.t_big_4 d  (cost=0.00..205601.02 rows=4166602 width=62) (actual time=0.163..7282.409 rows=3333333 loops=3)
                     Output: d.c3, d.c2
                     Buffers: shared hit=32 read=163903 dirtied=133935 written=133839
                     Worker 0: actual time=0.108..7244.071 rows=3297904 loops=1
                       Buffers: shared hit=9 read=54055 dirtied=44154 written=44122
                     Worker 1: actual time=0.034..7223.191 rows=3258559 loops=1
                       Buffers: shared hit=6 read=53413 dirtied=43464 written=43432
 Planning Time: 1.134 ms
 Execution Time: 405878.841 ms
(83 rows)

[local:/data/run/pg12]:5120 pg12@testdb=#

```
可以看到，執行計劃中的成本回歸一個正常的數值，演算法使用Hash Join。由於記憶體不足，PG把資料拆分為N份，使用臨時表來臨時快取Hash Table，使用不同的Batch來執行Join。

PostgreSQL DBA(155) - Develop（“大表”連線）

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