Ambigous Plan - Larger Table on Hash Side

[Narendra Pradeep U U]

Hi ,

    

      Recently I came across a case where the planner choose larger table on 
hash side. I am not sure whether it is an intended  behavior or we are missing 
something. 

  

     I have two tables (a and b) each with  single column in it. One table 'a' 
is large with around 30 million distinct rows and other table 'b' has merely 
70,000 rows with one-seventh (10,000) distinct rows. I have analyzed both the 
table.  But while joining both the table I get the larger table on hash side. 

tpch=# explain select b from b left join a on a = b;

                                               QUERY PLAN                       
                         

---------------------------------------------------------------------------------------------------------

 Hash Left Join  (cost=824863.75..950104.42 rows=78264 width=4)

   Hash Cond: (b.b = a.a)o

   ->  Foreign Scan on b  (cost=0.00..821.64 rows=78264 width=4)

         CStore File: /home/likewise-open/pg96/data/cstore_fdw/1818708/1849879

         CStore File Size: 314587

   ->  Hash  (cost=321721.22..321721.22 rows=30667722 width=4)

         ->  Foreign Scan on a  (cost=0.00..321721.22 rows=30667722 width=4)

               CStore File: 
/home/likewise-open/pg96/data/cstore_fdw/1818708/1849876

               CStore File Size: 123236206

(9 rows)






I would like to know the reason for choosing this plan and Is there a easy fix 
to prevent such plans (especially like this one where it choose a larger hash 
table) ?  



Thanks,

Pradeep