Here's how I "fixed" this issue. I ended up regenerating the shapefile
and dbf and adding a separate column with the grouping I desired. Now,
my map file selects from that column, and the syntax is much simpler (by
one or two orders of magnitude). I am happy with the results, however, I
didn't get the chance to try out all the other methods because of lack
of time.
Thanks again,
Adrian
Adrian Popa wrote:
Hello Steve,
I haven't tried out the simplified regex so I don't know if it will be
faster. I will try to test it as part of a speed test of the various
methods...
I'm not sure what you mean by writing a temporary set of geometries.
Do you mean adding an index to my data so that I can select it by a
different (grouping) field instead? Unfortunately I can't do that
because the same item can be part of 10-20 groups, so there would not
be an easy way to group items apart from duplicating them in the
shapefile/dbf. I'm not sure if there's a problem if the same feature
appears 12 times in the same shapefile.
In the end data reorganizing might be the fastest method available.
Problem is some items will belong to groups dinamically, so I will
have to implement a selection mechanism based on item id...
Regards,
Adrian
Steve Lime wrote:
Have you tried a simplified version of your regex? I think you can do:
EXPRESSION /^ITEM1|ITEM2|ITEM3|ITEM4$/
You might also consider writing a temporary set of geometries if a user will
continually display from
that set. In that case your overhead would be in managing the set of features
which would be higher
the first time but then very fast to render. Your dynamic portion of the
mapfile would reference the
temporary data.
Steve
On 7/14/2009 at 1:15 AM, in message <[email protected]>, Adrian Popa
<[email protected]> wrote:
Hello everyone,
Here's my problem: I'm trying to highlight segments from a line layer by
using an expression in a specific class. This portion of the mapfile is
dynamically generated and when it is done, it is sent to mapserver for
rendering.
My problem is that I have to select between 10 - 400 features at a time
and I noticed when I have a lot of features there is a severe
performance degradation in mapserver (takes a lot of time to render or
even times out).
Right now, my expression is built using regular expressions: something like:
*EXPRESSION /^ITEM1$|^ITEM2$|^ITEM3$|^ITEM4$/*
This works ok, but as I said has a performance penalty when you reach
~400 items. My data is selected from a shapefile layer which has about
5500 items.
Since I wouldn't be using the regular expressions at full capacity (I'm
matching the full name), I might rewrite the expression using something
like:
*EXPRESSION ( ([NAME]=="ITEM1") OR ([NAME]=="ITEM2") OR
([NAME]=="ITEM3") OR ([NAME]=="ITEM4") )*
From the documentation I see that:
/Regular expression with MapServer work similarly to string comparison,
but allow more complex operation. They are slower than pure string
comparisons, but might be still faster than logical expression. As with
the string comparison use regular expressions, a FILTERITEM or a
CLASSITEM has to defined, respectively.
/I would like to know if there is an efficient way of selecting a list
of elements from a layer, or what are your recommendations.
Also - have there been significant changes in performance for this issue
from mapserver 4.10 (I am now migrating to mapserver 5.4)?
Thanks,
Adrian
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