Greenplum , HAWQ outer join与motion问题讲解
作者
digoal
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。
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