<div dir="ltr">thanks.<br>yes it helps :)<br><br><div class="gmail_quote">On Tue, Jul 29, 2008 at 4:46 PM, Obe, Regina <span dir="ltr"><<a href="mailto:robe.dnd@cityofboston.gov">robe.dnd@cityofboston.gov</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">Haven't done any benchmarks, but I know the
following</font></span></div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">the ::text display you get is basically the string
representation of the native binary format that PostGIS actually stores the
geometries.</font></span></div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">It is not WKB nor WKT. Its a PostGIS proprietary
format if you will. I would guess casting from that format is the most
efficient since in theory there is very little processing that PostgreSQL needs
to do to reconstitute from that format.</font></span></div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">Keep in mind EWKT and WKT also may result in floating point
errors since they will round the decimals so be careful using those.
</font></span></div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">So your best formats to maintain precision (and not have
lossiness) are EWKB and the ::text representation (which I shall dubb the native
PostGIS binary notation (which I guess we can call light-weight something or
other)).</font></span></div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">Hope that helps,</font></span></div>
<div dir="ltr" align="left"><span><font color="#0000ff" size="2" face="Arial">Regina</font></span></div><br>
<div dir="ltr" align="left" lang="en-us">
<hr>
<font size="2" face="Tahoma"><b>From:</b>
<a href="mailto:postgis-users-bounces@postgis.refractions.net" target="_blank">postgis-users-bounces@postgis.refractions.net</a>
[mailto:<a href="mailto:postgis-users-bounces@postgis.refractions.net" target="_blank">postgis-users-bounces@postgis.refractions.net</a>] <b>On Behalf Of
</b>Willy-Bas Loos<br><b>Sent:</b> Tuesday, July 29, 2008 8:43 AM<br><b>To:</b>
<a href="mailto:postgis-users@postgis.refractions.net" target="_blank">postgis-users@postgis.refractions.net</a><br><b>Subject:</b> [postgis-users] optimal
textbased geometry format<br></font><br></div><div><div></div><div class="Wj3C7c">
<div></div>
<div dir="ltr">Sometimes i build queries inside plpgsql scripts, that i then run
using EXECUTE.<br>Is there a preferred way to include geometries in
those?<br><br>I use postgis 1.1 (for now). Are there are any changes in later
versions that influence this aspect of PostGIS?<br>The SRID is a factor for me,
so asText(the_geom) is no option.<br>I eiter use asEWKT(the_geom), or just
the_geom::text .<br>Just casting the geometry directly into text results in a
very long text value (what is the name of this format?). The length of the
string might be a network transfer drawback.<br>asEWKT results in human readable
format, possibly shorter, but it might require more processing power (back and
forth!). <br><br>I've tested both difference in length and performance, all on a
limited dataset.<br>LENGTH:<br>The difference in length varies greatly. I use
SRID 28992, which is a national grid that has coordinates in meters, so it is
possible to have coordinates with 0 decimal places (1 meter precision). Simple
geometries seem to benefit most from the WKT format. The EWKT is 40% of the
number of characters for some cases (square polygons from coordinates without
decimals). Most polygons score 50-60% of length, but in some cases the WKT
representation is up to 10% longer than the direct cast
(geom::text).<br>code:<br> --select length(the_geom::text),
length(asewkt(the_geom)),
round(((length(asewkt(the_geom))::float)/(length(the_geom::text))::float)*100)
as perc<br> select
avg(((length(asewkt(the_geom))::float)/(length(the_geom::text))::float)*100) as
perc<br> from atable<br> order by perc<br><br>PERFORMANCE:<br>I've
tested these "warm" (not the first run), and averaged the scores from 3
measurements each. The fluctuations were minimal.<br>The diference in
performance is small for converting from geometries to a text representation.
Casting directly is a factor 1.1 faster (90% of the time needed)<br>The other
way around is a diferent story. Converting a directly casted geometry back to a
geometry is a factor 6 faster than a EWKT string (16% of the time needed)!<br>I
hope my method (code below) is adequate?<br><br>code:<br> select
area(the_geom::text::geometry)<br> --select
area(asewkt(the_geom)::geometry)<br> from atable<br><br>Regarding my
tests, i would say that the format that results from a direct cast is the better
string representation for intra-application communication. Are there any
drawbacks?<br>Of course my test was limited to a small dataset on a windows pc
(mem 2GB, athlon 64 3200+) with postgis 1.1. Does anyone else have different
results/ideas about
this?<br><br>Cheers,<br><br>WBL<br><br><br></div></div></div></div>
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