perfecto, thank you for the explanation. - Deepak
On Thu, Oct 21, 2010 at 3:20 AM, Mathieu De Zutter <math...@dezutter.org>wrote: > On Thu, Oct 21, 2010 at 3:47 AM, DM <dm.a...@gmail.com> wrote: > > I was hoping the optimizer would do a join using index scan. > > > > Could some one please explain me why its not doing an index scan rather > than > > sequential scan . > > > A index scan would be probably slower here because you're asking for a > lot of rows. A lot of rows means a lot of I/O, and an index scan is > more I/O intensive (since it has to read the index too). If you limit > the result (by being more selective in your where clause, just like > you do in the first two queries), postgres will most likely switch to > index scan. > > You can see for yourself if index-scan would be faster in your case by > running the following command before "explain (analyze)": > > set enable_seqscan = off; > > BTW, try to use explain analyze instead of explain, that way you'll > see the actual timings too instead of just the planner estimates. > > Kind regards, > Mathieu >
