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To: kilin@cs.umd.edu
Cc: postgres@postgres.Berkeley.EDU
Subject: Re: Question on R-trees 
In-Reply-To: kilin's message of 15 Dec 94 10:22 EST
Date: Thu, 15 Dec 94 08:52:08 -0800
From: Mike Olson <mao@illustra.com>
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Resent-Date: Thu, 15 Dec 94 08:50:28 -0800
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kilin@cs.UMD.EDU writes:

> (1) Does it mean that everytime I query using the box operation (like <<, &<
>     etc.) the Rtree will be used to access the tuples? Can I force the
>     system to use the Rtree?

the query optimizer decides whether or not to use the index, based on
the expected size of the result set.  if the table is very small, then
using the index will be purely overhead, since you'll likely have to
touch every page in both the index and the base table.

if the table's large, then in general, postgres will (correctly) choose
to use the index.

> (2) Will the output be ordered in anyway beacuase of the use of the index?
>     I try to run a couple of querys and the output is ordered as the order
>     of inputting them. Is it supposed to be that way?

rtrees return data in an order that makes no sense to the user.  it
depends on the input order, the spatial distribution of boxes, and
the query rectangle.  don't try to figure it out.

> (3) (A dumb question), for the box datatype, is a box constant specified
>     by : "(xlow, ylow, xhigh, yhigh)"::box ?

yes.

> Thanks a lot

no problem, it was kind of fun.
					mike

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