Thanks for the information Tom. It solved my confusion. On Sun, Sep 18, 2011 at 10:03 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Craig Ringer <ring...@ringerc.id.au> writes: > > On 18/09/2011 5:51 PM, Melaka Gunasekara wrote: > >> Merge Full Join (cost=10000000074.40..10000000093.69 rows=1159 > width=286) > > >> Can you suggest why the merge join is being suggested when I have > >> turned it off ? > > > AFAIK SETting a join type to "off" really just increases the cost > > estimate so high that the planner will avoid using it where it has any > > alternative. In this case, it doesn't seem to think it has any other way > > to execute the query, or it thinks that any other way will be so > > incredibly, insanely slow that the merge join is still better. > > It's the first of those --- FULL joins are only implemented in the > mergejoin logic, not in hash or nestloop joins, so there is no other way > to do this query. (But as of 9.1, hash joins can do them too.) > > regards, tom lane > -- Best Regards, Melaka
