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

[Adrian Popa]

Just for reference:

I tried to select about 130 items using a regular expression in the form 
of EXPRESSION (/^ITEM1|ITEM2|ITEM3|...|ITEM130$/) and rendering took 
about 4.5-5 seconds.

I tried to select the exact same 130 items using "IN" syntax: EXPRESSION 
("[myfield]" IN "ITEM1,ITEM2,ITEM3,...,ITEM130") and the query took 
about 8.5-9 seconds.

The tests were run several times in the same conditions - so the 
relative results should be relevant.

So - performance favors regular expressions :)



Adrian Popa wrote:
> 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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>
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-- 
--- 
Adrian Popa
NOC Division
Network Engineer
Divizia Centrul National de Operare Retea
Departament Transport IP & Metro
Compartiment IP Core & Backbone
Phone: +40 21 400 3099

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