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Greenplum , HAWQ outer join与motion问题讲解

2016 Postgres大象会官方报名通道：点此报名

作者: digoal

日期: 2016-09-09

标签: PostgreSQL , HAWQ , Greenplum , OUTER JOIN , Motion

背景

Greenplum，HAWQ是分布式的数据库，在建表时，我们可以选择分布列，或者选择随机分布。

多个表做等值JOIN时，如果JOIN列为分布列，则不需要进行数据的重分布。

但是，如果使用的是OUTER JOIN，情况就不一样了，你可能会发现多个表进行outer join时，如果JOIN列都是HASH分布列，某些写法就可能导致需要重分布。

下面给大家分析一下原因。

创建几张测试表

其中tab1,tab2,tab3的bucketnum一致，分布列一致。

tab4,tab5,tab6的分布列与前面几张表一致，但是bucketnum不一致（所以显然HASH取模后的值也不一致）。

bucketnum默认是实体segments的6倍（意思是每台实体segment上跑6个虚拟segment），用户可以根据表的大小来调试，例如要发挥最大的计算能力，实际设置时可以设置为主机CPU核数的0.8，小表则建议设置为较小的值。

tab7与tab8为随机分布，对于随机分布的表，bucketnum设置会忽略，在实体segments之间随机分布(查看hdfs对应的文件也能看出端倪)。

postgres=# create table tab1(c1 int, c2 int, c3 text, c4 timestamp) with (bucketnum=6) distributed by(c1,c2);
postgres=# create table tab2(c1 int, c2 int, c3 text, c4 timestamp) with (bucketnum=6) distributed by(c1,c2);
postgres=# create table tab3(c1 int, c2 int, c3 text, c4 timestamp) with (bucketnum=6) distributed by(c1,c2);
postgres=# create table tab4(c1 int, c2 int, c3 text, c4 timestamp) with (bucketnum=10) distributed by(c1,c2);
postgres=# create table tab5(c1 int, c2 int, c3 text, c4 timestamp) with (bucketnum=12) distributed by(c1,c2);
postgres=# create table tab6(c1 int, c2 int, c3 text, c4 timestamp) with (bucketnum=3) distributed by(c1,c2);
postgres=# create table tab7(c1 int, c2 int, c3 text, c4 timestamp) with (bucketnum=3) distributed randomly;
postgres=# create table tab8(c1 int, c2 int, c3 text, c4 timestamp) with (bucketnum=6) distributed randomly;
postgres=# insert into tab7 select generate_series(1,1000000),generate_series(1,1000000),'test',now();
INSERT 0 1000000
postgres=# insert into tab8 select generate_series(1,1000000),generate_series(1,1000000),'test',now();
INSERT 0 1000000
postgres=# analyze tab7;
ANALYZE
postgres=# analyze tab8;
ANALYZE

下面开始几组测试，通过执行计划的motion node，观察query是否需要重分布，以及重分布那张表，重分布的键值是哪些。

测试1

HAWQ的hash分布取模值取决于bucket num，如果bucket num不一致，则JOIN时有一张表需要重分布

postgres=# explain select * from tab1 join tab6 on (tab1.c1=tab6.c1 and tab1.c2=tab6.c2);
                                              QUERY PLAN  
-----------------------------------------------------------------------------------------
 Gather Motion 6:1  (slice2; segments: 6)  (cost=0.00..862.00 rows=1 width=48)
   ->  Hash Join  (cost=0.00..862.00 rows=1 width=48)
         Hash Cond: tab1.c1 = tab6.c1 AND tab1.c2 = tab6.c2
         ->  Table Scan on tab1  (cost=0.00..431.00 rows=1 width=24)
         ->  Hash  (cost=431.00..431.00 rows=1 width=24)
               ->  Redistribute Motion 6:6  (slice1; segments: 6)  (cost=0.00..431.00 rows=1 width=24)
                                              -----    重分布tab6
                     Hash Key: tab6.c1, tab6.c2
                     ->  Table Scan on tab6  (cost=0.00..431.00 rows=1 width=24)
 Settings:  default_hash_table_bucket_number=6
 Optimizer status: PQO version 1.638
(10 rows)

测试2

如果JOIN的其中一个表为随机分布式，随机分布的表需要复制或按JOIN列进行重分布

postgres=# explain select * from tab6 join tab7 on (tab6.c1=tab7.c1 and tab6.c2=tab7.c2);
                                              QUERY PLAN  
----------------------------------------------------------------------------------------
 Gather Motion 3:1  (slice2; segments: 3)  (cost=0.00..862.00 rows=1 width=48)
   ->  Hash Join  (cost=0.00..862.00 rows=1 width=48)
         Hash Cond: tab6.c1 = tab7.c1 AND tab6.c2 = tab7.c2
         ->  Table Scan on tab6  (cost=0.00..431.00 rows=1 width=24)
         ->  Hash  (cost=431.00..431.00 rows=1 width=24)
               ->  Redistribute Motion 3:3  (slice1; segments: 3)  (cost=0.00..431.00 rows=1 width=24)
                                          ----重分布tab7
                     Hash Key: tab7.c1, tab7.c2
                     ->  Table Scan on tab7  (cost=0.00..431.00 rows=1 width=24)
 Settings:  default_hash_table_bucket_number=6
 Optimizer status: PQO version 1.638
(10 rows)

测试3

随机分布策略的表，在每个segment上只有一个bucket

postgres=# explain select count(*) from tab7;
                                     QUERY PLAN      
----------------------------------------------------------------------------------
 Aggregate  (cost=0.00..452.11 rows=1 width=8)
   ->  Gather Motion 1:1  (slice1; segments: 1)  (cost=0.00..452.11 rows=1 width=8)
         ->  Aggregate  (cost=0.00..452.11 rows=1 width=8)
               ->  Table Scan on tab7  (cost=0.00..450.25 rows=1000000 width=1)
 Settings:  default_hash_table_bucket_number=6
 Optimizer status: PQO version 1.638
(6 rows)

测试4

如果两个随机分布的表JOIN，数据需要在所有的实体segments(每个datanode对应一个实体segment)上按JOIN列重分布

postgres=# explain select * from tab8 join tab7 on (tab8.c1=tab7.c1 and tab8.c2=tab7.c2);
                                                 QUERY PLAN
----------------------------------------------------------------------------------------
 Gather Motion 1:1  (slice3; segments: 1)  (cost=0.00..2151.49 rows=10000 width=42)
   ->  Hash Join  (cost=0.00..2148.64 rows=10000 width=42)
         Hash Cond: tab8.c1 = tab7.c1 AND tab8.c2 = tab7.c2
         ->  Redistribute Motion 1:1  (slice1; segments: 1)  (cost=0.00..555.04 rows=1000000 width=21)
                                              ----------------重分布tab8
               Hash Key: tab8.c1, tab8.c2
               ->  Table Scan on tab8  (cost=0.00..450.25 rows=1000000 width=21)
         ->  Hash  (cost=555.04..555.04 rows=1000000 width=21)
               ->  Redistribute Motion 1:1  (slice2; segments: 1)  (cost=0.00..555.04 rows=1000000 width=21)
                                             ------------------重分布tab7
                     Hash Key: tab7.c1, tab7.c2
                     ->  Table Scan on tab7  (cost=0.00..450.25 rows=1000000 width=21)
 Settings:  default_hash_table_bucket_number=6
 Optimizer status: PQO version 1.638
(12 rows)

测试5

outer join，2张表的outer join，如果JOIN列就是分布列，不需要重分布

postgres=# explain select * from tab1 left join tab2 on (tab1.c1=tab2.c1 and tab1.c2=tab2.c2);
                                  QUERY PLAN            
-------------------------------------------------------------------------------
 Gather Motion 6:1  (slice1; segments: 6)  (cost=0.00..862.00 rows=2 width=48)
   ->  Hash Left Join  (cost=0.00..862.00 rows=1 width=48)
         Hash Cond: tab1.c1 = tab2.c1 AND tab1.c2 = tab2.c2
         ->  Table Scan on tab1  (cost=0.00..431.00 rows=1 width=24)
         ->  Hash  (cost=431.00..431.00 rows=1 width=24)
               ->  Table Scan on tab2  (cost=0.00..431.00 rows=1 width=24)
 Settings:  default_hash_table_bucket_number=6
 Optimizer status: PQO version 1.638
(8 rows)

测试6

outer join，3张表的outer join，需要注意JOIN的条件

1.tab1与tab2 left join后，关联不上时tab2可能会返回一些NULL值

因此再次与tab3 join时，如果JOIN条件是tab2与tab3，则不能圈定在虚拟segment内完成tab2与tab3的JOIN，必须要对tab1与tab2的outer JOIN中间结果进行重分布，对齐tab3的分布策略，再进行JOIN。

但是实际上，并不需要重分布，因为null和null比较返回的还是null，所以null实际上不需要重分布到一起去JOIN，本条SQL，对于HAWQ的优化器来说，是有优化余地的。

postgres=# explain select * from tab1 left join tab2 on (tab1.c1=tab2.c1 and tab1.c2=tab2.c2) 
                            left join tab3 on (tab2.c1=tab3.c1 and tab2.c2=tab3.c2);
                                           QUERY PLAN     
--------------------------------------------------------------------------------------------
 Gather Motion 6:1  (slice2; segments: 6)  (cost=0.00..1293.00 rows=3 width=72)
   ->  Hash Left Join  (cost=0.00..1293.00 rows=1 width=72)
         Hash Cond: tab2.c1 = tab3.c1 AND tab2.c2 = tab3.c2
         ->  Redistribute Motion 6:6  (slice1; segments: 6)  (cost=0.00..862.00 rows=1 width=48)
               Hash Key: tab2.c1, tab2.c2
               ->  Hash Left Join  (cost=0.00..862.00 rows=1 width=48)
                     Hash Cond: tab1.c1 = tab2.c1 AND tab1.c2 = tab2.c2
                     ->  Table Scan on tab1  (cost=0.00..431.00 rows=1 width=24)
                     ->  Hash  (cost=431.00..431.00 rows=1 width=24)
                           ->  Table Scan on tab2  (cost=0.00..431.00 rows=1 width=24)
         ->  Hash  (cost=431.00..431.00 rows=1 width=24)
               ->  Table Scan on tab3  (cost=0.00..431.00 rows=1 width=24)
 Settings:  default_hash_table_bucket_number=6
 Optimizer status: PQO version 1.638
(14 rows)

2.这样的条件下，则不需要重分布，因为第一次LEFT JOIN后，TAB1不会产生空值，使用tab1再与tab3进行join也不需要重分布。

postgres=# explain select * from tab1 left join tab2 on (tab1.c1=tab2.c1 and tab1.c2=tab2.c2) 
                                left join tab3 on (tab1.c1=tab3.c1 and tab1.c2=tab3.c2);
                                   QUERY PLAN       
---------------------------------------------------------------------------------
 Gather Motion 6:1  (slice1; segments: 6)  (cost=0.00..1293.00 rows=3 width=72)
   ->  Hash Left Join  (cost=0.00..1293.00 rows=1 width=72)
         Hash Cond: tab1.c1 = tab3.c1 AND tab1.c2 = tab3.c2
         ->  Hash Left Join  (cost=0.00..862.00 rows=1 width=48)
               Hash Cond: tab1.c1 = tab2.c1 AND tab1.c2 = tab2.c2
               ->  Table Scan on tab1  (cost=0.00..431.00 rows=1 width=24)
               ->  Hash  (cost=431.00..431.00 rows=1 width=24)
                     ->  Table Scan on tab2  (cost=0.00..431.00 rows=1 width=24)
         ->  Hash  (cost=431.00..431.00 rows=1 width=24)
               ->  Table Scan on tab3  (cost=0.00..431.00 rows=1 width=24)
 Settings:  default_hash_table_bucket_number=6
 Optimizer status: PQO version 1.638
(12 rows)

3.如果第三张关联表是JOIN条件，而非OUTER JOIN，同样不需要重分布。

postgres=# explain select * from tab1 left join tab2 on (tab1.c1=tab2.c1 and tab1.c2=tab2.c2) 
                             join tab3 on (tab2.c1=tab3.c1 and tab2.c2=tab3.c2);
                                               QUERY PLAN   
--------------------------------------------------------------------------------------------
 Gather Motion 6:1  (slice1; segments: 6)  (cost=0.00..1293.00 rows=1 width=72)
   ->  Hash Join  (cost=0.00..1293.00 rows=1 width=72)
         Hash Cond: tab2.c1 = tab3.c1 AND tab2.c2 = tab3.c2 AND tab1.c1 = tab3.c1 AND tab1.c2 = tab3.c2
         ->  Hash Join  (cost=0.00..862.00 rows=1 width=48)
               Hash Cond: tab1.c1 = tab2.c1 AND tab1.c2 = tab2.c2
               ->  Table Scan on tab1  (cost=0.00..431.00 rows=1 width=24)
               ->  Hash  (cost=431.00..431.00 rows=1 width=24)
                     ->  Table Scan on tab2  (cost=0.00..431.00 rows=1 width=24)
         ->  Hash  (cost=431.00..431.00 rows=1 width=24)
               ->  Table Scan on tab3  (cost=0.00..431.00 rows=1 width=24)
 Settings:  default_hash_table_bucket_number=6
 Optimizer status: PQO version 1.638
(12 rows)

4.只有JOIN时，也不需要考虑重分布。

postgres=# explain select * from tab1 join tab2 on (tab1.c1=tab2.c1 and tab1.c2=tab2.c2) 
                   join tab3 on (tab2.c1=tab3.c1 and tab2.c2=tab3.c2);
                                               QUERY PLAN 
---------------------------------------------------------------------------------------
 Gather Motion 6:1  (slice1; segments: 6)  (cost=0.00..1293.00 rows=1 width=72)
   ->  Hash Join  (cost=0.00..1293.00 rows=1 width=72)
         Hash Cond: tab2.c1 = tab3.c1 AND tab2.c2 = tab3.c2 AND tab1.c1 = tab3.c1 AND tab1.c2 = tab3.c2
         ->  Hash Join  (cost=0.00..862.00 rows=1 width=48)
               Hash Cond: tab1.c1 = tab2.c1 AND tab1.c2 = tab2.c2
               ->  Table Scan on tab1  (cost=0.00..431.00 rows=1 width=24)
               ->  Hash  (cost=431.00..431.00 rows=1 width=24)
                     ->  Table Scan on tab2  (cost=0.00..431.00 rows=1 width=24)
         ->  Hash  (cost=431.00..431.00 rows=1 width=24)
               ->  Table Scan on tab3  (cost=0.00..431.00 rows=1 width=24)
 Settings:  default_hash_table_bucket_number=6
 Optimizer status: PQO version 1.638
(12 rows)

小结

1. 随机分布的表与随机分布的表进行JOIN时，可能无法充分利用计算资源，因为每个物理节点只能用到一个核。

2. 随机分布的表与哈希分布的表JOIN时，会根据实际情况，重分布，并行计算。（如果哈希分布的表bucketnum较多，这种QUERY也能用上多核JOIN）。

3. outer join时，如果多次进行，请注意实际的场景逻辑，建议在JOIN时过滤，而不是JOIN完后过滤NULL，以避免重分布。

2016 Postgres大象会官方报名通道：http://www.huodongxing.com/event/8352217821400

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