[mapserver-users] Performance in
regular expressions or analternative way to select a list of features
adrian_gh.popa at romtelecom.ro
Tue Jul 21 04:32:25 EDT 2009
Just for reference:
I tried to select about 130 items using a regular expression in the form
of EXPRESSION (/^ITEM1|ITEM2|ITEM3|...|ITEM130$/) and rendering took
about 4.5-5 seconds.
I tried to select the exact same 130 items using "IN" syntax: EXPRESSION
("[myfield]" IN "ITEM1,ITEM2,ITEM3,...,ITEM130") and the query took
about 8.5-9 seconds.
The tests were run several times in the same conditions - so the
relative results should be relevant.
So - performance favors regular expressions :)
Adrian Popa wrote:
> Here's how I "fixed" this issue. I ended up regenerating the shapefile
> and dbf and adding a separate column with the grouping I desired. Now,
> my map file selects from that column, and the syntax is much simpler
> (by one or two orders of magnitude). I am happy with the results,
> however, I didn't get the chance to try out all the other methods
> because of lack of time.
> Thanks again,
> Adrian Popa wrote:
>> Hello Steve,
>> I haven't tried out the simplified regex so I don't know if it will
>> be faster. I will try to test it as part of a speed test of the
>> various methods...
>> I'm not sure what you mean by writing a temporary set of geometries.
>> Do you mean adding an index to my data so that I can select it by a
>> different (grouping) field instead? Unfortunately I can't do that
>> because the same item can be part of 10-20 groups, so there would not
>> be an easy way to group items apart from duplicating them in the
>> shapefile/dbf. I'm not sure if there's a problem if the same feature
>> appears 12 times in the same shapefile.
>> In the end data reorganizing might be the fastest method available.
>> Problem is some items will belong to groups dinamically, so I will
>> have to implement a selection mechanism based on item id...
>> Steve Lime wrote:
>>> Have you tried a simplified version of your regex? I think you can do:
>>> EXPRESSION /^ITEM1|ITEM2|ITEM3|ITEM4$/
>>> 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.
>>>>>> 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
>>>> 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
>>>> *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)?
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