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

[Adrian Popa]

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$/
>
> You might also consider writing a temporary set of geometries if a user will continually display from
> that set. In that case your overhead would be in managing the set of features which would be higher
> the first time but then very fast to render. Your dynamic portion of the mapfile would reference the
> temporary data.
>
> Steve
>
>   
>>>> On 7/14/2009 at 1:15 AM, in message <4A5C2277.80204 at romtelecom.ro>, Adrian Popa
>>>>         
> <adrian_gh.popa at romtelecom.ro> wrote:
>   
>> Hello everyone,
>>
>> Here's my problem: I'm trying to highlight segments from a line layer by 
>> using an expression in a specific class. This portion of the mapfile is 
>> dynamically generated and when it is done, it is sent to mapserver for 
>> rendering.
>> My problem is that I have to select between 10 - 400 features at a time 
>> and I noticed when I have a lot of features there is a severe 
>> performance degradation in mapserver (takes a lot of time to render or 
>> even times out).
>> Right now, my expression is built using regular expressions: something like:
>> *EXPRESSION /^ITEM1$|^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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