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Don Burleson Blog 







driving_site hint tips

Oracle Database Tips by Donald BurlesonDecember 16,  2015


Question:  I have a distributed query and it's doing a full table scan on the remote table.  However, when I add a "driving_site" hint, the query runs fine.  Why is a driving_site hint required in order for Oracle to use the remote index?

Answer:  In a true distributed query, the optimization is done on the sending site.  Because your local site may not have access to the CBO statistics on the remote site, Oracle "punts" and does a full-table scan, and this is likely the cause of your full scan.

The driving_site hint forces query execution to be done at a different site than the initiating instance. This is done when the remote table is much larger than the local table and you want the work (join, sorting) done remotely to save the back-and-forth network traffic.

For example, we use the driving_site hint to force the "work" to be done on the site where the huge table resides:

select /*+DRIVING_SITE(h)*/
  tiny_table        t,
  huge_table@remote h
   t.deptno = h.deptno;

Note:  The driving_site hint will not work with CTAS (create table as select) and with create materialized view syntax because they are DDL and these operations must take place on the original instance.  See MOSC Note 825677.1 for details.



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