Thread: Re: [GENERAL] GiST: Need ideas on how to minimise data in a GiST index

Hi Tom,

> "Mark Cave-Ayland" <m.cave-ayland@webbased.co.uk> writes:
> > However, each database column also has a spatial reference system
> > identifier (SRID) that specifies the coordinate system the
> geometry is
> > in. What should happen is that given two geometries or
> bounding boxes,
> > an error should be generated whenever the SRIDs do not match.
>
> Offhand I'm not sure why you think the index should solve
> this.  It seems more like grist for a CHECK constraint
> applied to the table.

Yes, there is already a check constraint on the column that quite nicely
handles inserts/deletions etc. The problem occurs when someone tries to
query an area in a form like:

select * from geomtable where geom && BOX2D('1.0000 1.000, 2.000 2.000',
-2)

(where say -2 is the SRID and geom has a SRID of -1). In this case the
BOX2D (which is the area you are trying to query) gets cast into a BOX
based on its bounding box to perform the query with no way of
recognising the SRIDs are different :(

> One way to attack it without bloating the index is to
> consider the index as lossy, meaning that the actual
> geometric operator must be reapplied on each row identified
> by the index.  Then the operator can compare SRIDs, but you
> need not store SRIDs in the index.

This sounds exactly as if it would solve the problem. Have you got any
pointers to documentation on this, and more specifically on the
PostgreSQL implementation of GiST? (I'm guessing this is a GiST only
extension). Has it been available since PostgreSQL 7.1 aswell?


Cheers,

Mark.


---

Mark Cave-Ayland
Webbased Ltd.
Tamar Science Park
Derriford
Plymouth
PL6 8BX
England

Tel: +44 (0)1752 764445
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"Mark Cave-Ayland" <m.cave-ayland@webbased.co.uk> writes:
> This sounds exactly as if it would solve the problem. Have you got any
> pointers to documentation on this, and more specifically on the
> PostgreSQL implementation of GiST? (I'm guessing this is a GiST only
> extension). Has it been available since PostgreSQL 7.1 aswell?

No, lossiness is a generic index property.  There's been some level of
support for it since Berkeley days, although we improved the flexibility
in 7.2 --- we now think of lossiness as a property associated with
specific operators rather than a whole index.  That would probably not
matter for this particular situation, since you'd be wanting to mark all
the operators lossy anyway.

You can read about this stuff in the PG Programmer's Guide,
"Interfacing Extensions To Indexes".  I'd start with the 7.3 version
and then work backwards.

            regards, tom lane