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

[Adrian Popa]

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