[mapserver-users] Performance in regular expressions or an alternative way to select a list of features

[Adrian Popa]

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:* mapserver-users-bounces at lists.osgeo.org
>     [mailto:mapserver-users-bounces at lists.osgeo.org] *On Behalf Of
>     *Adrian Popa
>     *Sent:* Tuesday, July 14, 2009 1:15 AM
>     *To:* mapserver-users at lists.osgeo.org
>     *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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