Thanks for the information Tom.
It solved my confusion.
On Sun, Sep 18, 2011 at 10:03 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Craig Ringer <ringerc(at)ringerc(dot)id(dot)au> writes:
> > On 18/09/2011 5:51 PM, Melaka Gunasekara wrote:
> >> Merge Full Join (cost=10000000074.40..10000000093.69 rows=1159
> >> 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
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