[postgis-devel] Re: [HACKERS] join selectivity

[strk]

On Mon, Dec 13, 2004 at 12:16:05PM -0500, Tom Lane wrote:
> "Mark Cave-Ayland" <m.cave-ayland at webbased.co.uk> writes:
> > For a query like this:
> > 
> > SELECT id FROM table1, table2 
> > WHERE table1.geom && table2.geom;
> > 
> > RESTRICT selectivity is invoked twice and
> > JOIN selectivity is invoked once.
> 
> Hm, are you testing in a context where both tables have indexes that are
> relevant to the && operator?

Single index relevant to the && operator makes 2 calls to RESTRICT.
Double index (one for each table) makes 4 calls to RESTRICT.
In both cases JOIN is called once.

--strk;

> The estimated join result size is computed from the join selectivity
> estimate for the && operator.  I was about to say that restriction
> selectivity wouldn't be used at all, but on second thought I believe
> that it would be invoked while considering nestloop with inner indexscan
> plans.  That is, we'd consider
> 
> 	NestLoop
> 		Seq Scan on table2
> 		Indexscan on table1
> 			IndexCond: table1.geom && outer.geom
> 
> and to determine the estimated cost of each indexscan, we would invoke
> restriction selectivity for &&, with varRelid referencing table1.
> Given this call you are supposed to treat table2.geom as a constant of
> uncertain value, so the thing is semantically sensible as a restriction
> clause for table1 (whether you can produce a really good estimate is
> another question :-().
> 
> Similarly, we'd consider the reverse plan with table1 as outer, and
> that would give rise to another restriction selectivity check with
> varRelid = table2.
> 
> >> (2) Is JOIN selectivity a fraction of table2 X table1
> >> records ?
> 
> Yes.  Similarly restriction selectivity is a fraction of records in the
> table under consideration.
> 
> 			regards, tom lane
> 
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