[postgis-users] Documenting performance tip

[Markus Schaber]

Hi, strk,

strk at refractions.net schrieb:

> I see. Well, we can use Envelope(the_geom) then.
> It's OGC-compliant and retains SRID.
>
>   SELECT addGeometryColumn('mydb','mytable','bbox','4326','GEOMETRY','2');
>   UPDATE mytable set bbox = Envelope(geom_column);

This query is a lot more readable, its a pity that I did not have that idea.

But it seems to suffer from the box2d precision loss, I get 15 digit
output on lwgeom, while hwgeom and my query return the original
ordinates with 6-7 digits. The question is whether this does hurt, as
the && index operation also uses the tweaked coordinates.

> I'm not sure about dimensions. Currently both box3d->geometry cast and
> Envelope return a 2d geometry, but both could change in the future.
> Would a ZMpolygon be a problem ?

I do not know if any user will actually use measured geometries in a &&
query. Does && honour Z and M in any way? If not, it is fine to ignore
it in the bbox column.

If yes, we should try to preserve it in our bbox column, or document
that the sematics are changed.

My personal opinion: The whole thing is just only a workaround for a
hopefully soon-to-be fixed query planner problem that bites in a rather
strange corner case. I do not think that we should spend too much energy
polishing it.

Markus


--
markus schaber | dipl. informatiker
logi-track ag | rennweg 14-16 | ch 8001 zürich
phone +41-43-888 62 52 | fax +41-43-888 62 53
mailto:schabios at logi-track.com | www.logi-track.com
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