From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200512171643.jBHGhFA04982@candle.pha.pa.us>
Subject: Re: Should Oracle outperform PostgreSQL on a complex
In-Reply-To: <1134810953.2964.117.camel@localhost.localdomain>
To: Simon Riggs <simon@2ndquadrant.com>
Date: Sat, 17 Dec 2005 11:43:15 -0500 (EST)
CC: =?ISO-8859-1?Q?P=E5l_Stenslet?= <paal.stenslet@exie.com>,
	pgsql-performance@postgresql.org
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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.


---------------------------------------------------------------------------

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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