How are star joins different from what we do now?
---------------------------------------------------------------------------
Simon Riggs wrote:
> On Thu, 2005-12-08 at 12:26 +0100, P?l Stenslet wrote:
> > I'm currently benchmarking several RDBMSs with respect to analytical
> > query performance on medium-sized multidimensional data sets. The data
> > set contains 30,000,000 fact rows evenly distributed in a
> > multidimensional space of 9 hierarchical dimensions. Each dimension
> > has 8000 members.
>
> > I have established similar conditions for the query in PostgreSQL, and
> > it runs in about 30 seconds. Again the CPU utilization is high with no
> > noticable I/O. The query plan is of course very different from that of
> > Oracle, since PostgreSQL lacks the bitmap index merge operation. It
> > narrows down the result one dimension at a time, using the
> > single-column indexes provided. It is not an option for us to provide
> > multi-column indexes tailored to the specific query, since we want
> > full freedom as to which dimensions each query will use.
>
> > Are these the results we should expect when comparing PostgreSQL to
> > Oracle for such queries, or are there special optimization options for
> > PostgreSQL that we may have overlooked? (I wouldn't be suprised if
> > there are, since I spent at least 2 full days trying to trigger the
> > star optimization magic in my Oracle installation.)
>
> Yes, I'd expect something like this right now in 8.1; the numbers stack
> up to PostgreSQL doing equivalent join speeds, but w/o star join.
>
> You've confused the issue here since:
> - Oracle performs star joins using a bit map index transform. It is the
> star join that is the important bit here, not the just the bitmap part.
> - PostgreSQL does actually provide bitmap index merge, but not star join
> (YET!)
>
> [I've looked into this, but there seem to be multiple patent claims
> covering various aspects of this technique, yet at least other 3 vendors
> manage to achieve this. So far I've not dug too deeply, but I understand
> the optimizations we'd need to perform in PostgreSQL to do this.]
>
> Best Regards, Simon Riggs
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: In versions below 8.0, the planner will ignore your desire to
> choose an index scan if your joining column's datatypes do not
> match
>
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
