2011/1/18 masterchief <esi...@theiqgroup.com> > > > Tom Lane wrote: > > > > The only really effective way the planner knows to optimize an > > "IN (sub-SELECT)" is to turn it into a semi-join, which is not possible > > here because of the unrelated OR clause. You might consider replacing > > this with a UNION of two scans of "contexts". (And yes, I know it'd be > > nicer if the planner did that for you.) > > In moving our application from Oracle to Postgres, we've discovered that a > large number of our reports fall into this category. If we rewrite them as > a UNION of two scans, it would be quite a big undertaking. Is there a way > to tell the planner explicitly to use a semi-join (I may not grasp the > concepts here)? If not, would your advice be to hunker down and rewrite > the > queries? > > You can try "exists" instead of "in". Postgresql likes exists better. Alternatively, you can do something like "set enable_seqscan=false". Note that such set is more like a hammer, so should be avoided. If it is the only thing that helps, it can be set right before calling query and reset to default afterwards. --
Best regards, Vitalii Tymchyshyn
