Re: Optomizing left outer joins

Josh Berkus <josh@agliodbs.com> writes:
> FWIW, outer joins are slower than regular joins on all RDBMSs I've tested -- 
> often up to 5 times slower.   I'm not sure whether this is just the planner 
> restricitons inherent in an outer join, or whether this is something about 
> the required join algorithm itself.

There isn't anything fundamentally slower about left join vs plain join
(unless, perhaps, the left join has to generate a really large number
of null-extended rows that the plain join would not generate).  My money
is on the left join having prevented some optimization that could be
applied in the plain-join case.  But since we have not been shown any
exact details of the tables or the query, it's impossible to do more
than wager...
        regards, tom lane