Left outer join on linked server

Question

Hi,
I am migrating large amounts of data from Oracle database to SQL Server database, through a linked server.
When I use LEFT JOIN, the SQL server execution plan shows that the query that merges tables is not executed remotely (i.e. the data from both tables that I am trying to join is fetched on SQL server and joined on SQL server instead of being joined on Oracle and fetched after that), while queries using INNER JOIN fully execute on the remote server. I am trying to join remote Oracle database tables and the result of the join is supposed to be populated in a local table.This LEFT JOIN issue significantly enlarges the amount of a query execution time.
Can someone explain why is this so and suggest a solution? I tried using OPENQUERY but the results are not fetched properly because of the collation.
Thanks in advance!

Answer 1

with t1 as (
            select 1 id,to_date('1-Jan-12','dd-mon-rr') dateA from dual union all
            select 1,to_date('1-Feb-12','dd-mon-rr') from dual union all
            select 1,to_date('1-Mar-12','dd-mon-rr') from dual union all
            select 2,to_date('1-Feb-12','dd-mon-rr') from dual union all
            select 2,to_date('1-Mar-12','dd-mon-rr') from dual union all
            select 2,to_date('1-Apr-12','dd-mon-rr') from dual
     t2 as (
            select 1 id,to_date('1-Feb-12','dd-mon-rr') dateB,'a' valB from dual union all
            select 1,to_date('1-Mar-12','dd-mon-rr'),'b' from dual union all
            select 2,to_date('1-Feb-12','dd-mon-rr'),'c' from dual union all
            select 2,to_date('1-Mar-12','dd-mon-rr'),'d' from dual
select  t1.id,
        dateA,
        max(dateB) dateB,
        max(valB) keep(dense_rank last order by dateB) valB
  from      t1
        left join
            t2
          on     t2.id = t1.id
             and
                 dateB <= dateA
  group by t1.id,
           dateA
  order by id,
           dateA
        ID DATEA     DATEB     V
         1 01-JAN-12
         1 01-FEB-12 01-FEB-12 a
         1 01-MAR-12 01-MAR-12 b
         2 01-FEB-12 01-FEB-12 c
         2 01-MAR-12 01-MAR-12 d
         2 01-APR-12 01-MAR-12 d
6 rows selected.
SQL> SY.