Hi,
for a number of reasons I'd like to be able to analyze a filter and
optimize out functions when possible.
The main driver behind this is that none of the stores support functions
natively (they could, but they not) and thus the filters are not fully encoded
down to the database, with much performance detriment.
A typical case is the enviroment function, as soon as it's used the filters
using it stop being sent to the database, which is of course a performance
killer. It would be nice to recognize that the function is not using any
feature attribute, so it can be evaluated at the beginning of the data
access and optimized out into a literal.
Now, generically speaking the "optimize out" test is just analyzing
the function arguments and see if there is any usage of the feature
attributes: if there is none, the function is static, right?
Err... almost.
A function such as "random" is not going to return the same value
twice: that's a purely dynamic function.
A function such as "env" is going to return the same value during
all the data access, well, at least unless something else changes
the enviroment map _during_ the data access.
What I would like to have is some sort of DynamicFunction marker
interface that tells me if a function is dynamic or not. Maybe a
bit more than just a marker:
interface DynamicFunction {
/**
* Returns true if a function is going to return the same
* value during a data access operation unless
* someone changes its configuration
*/
boolean requestStable();
}
Random would implement that interface and return false,
env would implement it and return true.
Stores, in particular JDBC ones, would use a visitor
that optimizes out all functions not using attribute values
that are not dynamic, or that are dynamic but request stable.
This would make the usage of env functions better,
and would solve an issue reported by Mike P. recently
that the renderer is optimizing out a random() call into
the symbolizers be optimized out and return the same
value all over the render.
Opinions?
Cheers
Andrea
-----------------------------------------------------
Ing. Andrea Aime
Senior Software Engineer
GeoSolutions S.A.S.
Via Poggio alle Viti 1187
55054 Massarosa (LU)
Italy
phone: +39 0584962313
fax: +39 0584962313
http://www.geo-solutions.it
http://geo-solutions.blogspot.com/
http://www.linkedin.com/in/andreaaime
http://twitter.com/geowolf
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