Hello David,
I wasn't aware of the "IN" syntax. It looks cleaner, and hopefully will
be better. I think in the end I will try all sollutions and see which
one is the fastest for a given (large) expression.
Thangs again,
Adrian
Fawcett, David wrote:
Adrian,
One method that I have used is to use an IN statement in my expression
and then use a variable to populate the list of IDs in the statement.
I create a class like this:
CLASS
NAME "Low"
EXPRESSION ('[COUNTY_FIP]' in '%group1%')
OUTLINECOLOR 0 0 0
COLOR 255 204 204
END
And then in the URL calling the map, I include &group1=27001,27003,27005
I actually use this with five classes (five different URL vars) to
create a thematic map of counties based entirely on data passed in
through the URL.
I have no idea on how the performance of this method compares to what
you have done, but it might be worth a try. There are only 87
counties in Minnesota, so that is the largest number it gets, but the
performance isn't bad.
David.
-----Original Message-----
*From:* [email protected]
[mailto:[email protected]] *On Behalf Of
*Adrian Popa
*Sent:* Tuesday, July 14, 2009 1:15 AM
*To:* [email protected]
*Subject:* [mapserver-users] Performance in regular expressions or
an alternative way to select a list of features
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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