OK, so while our bitmap scan allows multiple indexes to be joined to get
to heap rows, a star joins allows multiple dimensions _tables_ to be
joined to index into a larger main fact table --- interesting.
Added to TODO:
* Allow star join optimizations
While our bitmap scan allows multiple indexes to be joined to get
to heap rows, a star joins allows multiple dimension _tables_ to
be joined to index into a larger main fact table. The join is
usually performed by either creating a cartesian product of all
the dimmension tables and doing a single join on that product or
using subselects to create bitmaps of each dimmension table match
and merge the bitmaps to perform the join on the fact table.
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Simon Riggs wrote:
> On Fri, 2005-12-16 at 23:28 -0500, Bruce Momjian wrote:
> > How are star joins different from what we do now?
>
> Various ways of doing them, but all use plans that you wouldn't have
> come up with via normal join planning.
>
> Methods:
> 1. join all N small tables together in a cartesian product, then join to
> main Large table once (rather than N times)
> 2. transform joins into subselects, then return subselect rows via an
> index bitmap. Joins are performed via a bitmap addition process.
>
> You can fake (1) yourself with a temporary table, and the basics for (2)
> are now in place also.
>
> The characteristics of these joins make them suitable for large Data
> Warehouses with Fact-Dimension style designs.
>
> Many RDBMS have this, but we need to be careful of patent claims. I'm
> sure there's a way through that, but I'm not looking for it yet. Anybody
> else wishing to assist with a detailed analysis would be much
> appreciated.
>
> Best Regards, Simon Riggs
>
--
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