postgres-xl left join 执行时间太长

Question

Postgres-XL 9.5r1.6 consists of a gtm, a coordinator and two datanodes.

There are three tables a , b and c which implements a many-to-many relationship:

create table a(id int, name text, uid int) distribute by hash(uid);
create table b(id int, name text, uid int) distribute by hash(uid);
create table c(id int, aname text, bname text, uid int) distribute by hash(uid);

when run following query on coordinator it takes inexplicable time of 20000 milliseconds ! but on either datanodes execution time is hardly more than 20 milliseconds .

select a.name, b.name

from 
       a left join c
       on a.name=c.aname

          left join b
          on c.bname=b.name
where
       a.name='cf82c96b77b8aa5277da6d55c4e4e66e';

explain plan on coordinator:

Remote Subquery Scan on all (dn_1,dn_2)  (cost=8.33..17.78 rows=1 width=66)


 ->  Nested Loop Left Join  (cost=8.33..17.78 rows=1 width=66)
         Join Filter: ((a.name)::text = (c.aname)::text)
         ->  Remote Subquery Scan on all (dn_1,dn_2)  (cost=100.15..108.21 rows=1 width=33)
               Distribute results by H: name
               ->  Index Only Scan using code_idx on a  (cost=0.15..8.17 rows=1 width=33)
                     Index Cond: (name = 'cf82c96b77b8aa5277da6d55c4e4e66e'::text)
         ->  Materialize  (cost=108.18..109.72 rows=1 width=115)
               ->  Remote Subquery Scan on all (dn_1,dn_2)  (cost=108.18..109.72 rows=1 width=115)
                     Distribute results by H: aname
                     ->  Hash Right Join  (cost=8.18..9.60 rows=1 width=115)
                           Hash Cond: ((b.name)::text = (c.bname)::text)
                           ->  Remote Subquery Scan on all (dn_1,dn_2)  (cost=100.00..102.44 rows=30 width=33)
                                 Distribute results by H: name
                                 ->  Seq Scan on b  (cost=0.00..1.30 rows=30 width=33)
                           ->  Hash  (cost=108.41..108.41 rows=1 width=244)
                                 ->  Remote Subquery Scan on all (dn_1,dn_2)  (cost=100.15..108.41 rows=1 width=244)
                                       Distribute results by H: bname
                                       ->  Index Only Scan using code_idxcfc on c  (cost=0.15..8.17 rows=1 width=244)
                                             Index Cond: (aname = 'cf82c96b77b8aa5277da6d55c4e4e66e'::text)

some other guy already hit this problem and asked here but with no answer or hint. I'm just hoping this time the question gets some insight.

ps: I tried to fill the three tables in a way that related rows from a and b which form table c only come from same datanode. But the execution time showed no improvment. Other point worth noting is that when condition in where clause ( a.name='cf82c96b77b8aa5277da6d55c4e4e66e' ) is always false, then the execution time drop low less than few milliseconds.

Answer 1

For this query:

select a.name, b.name
from a left join
     c
     on a.name = c.aname left join
     b
     on c.bname = b.name
where a.name = 'cf82c96b77b8aa5277da6d55c4e4e66e';

You want indexes on a(name) , b(name) , and c(name) . Your partitions are not going to help this query and you should only keep them if the tables are really big.

Answer 2

This is due to the nested loop, set it to false. XL will use hash join and then it will return results fastly