[mapserver-dev] [performance] mapserver list expressions

[thomas bonfort]

While profiling the openstreetmap renderings, I found a relatively low
hanging performance speedup related to expressions when doing class
filtering. The context relates to when you'd need to apply a given CLASS to
multiple attribute values.
Currently, to apply a single class to multiple values of an attribute, you
can either use regular expressions, or use the IN operator, like this:

LAYER
  ...
  CLASSITEM "type"
  CLASS
    EXPRESSION /primary|secondary|tertiary/  #regular expression on
CLASSITEM
    EXPRESSION ("[type]" IN "primary,secondary,tertiary") #"complex" parser
expression
    ...
  END
END

Both methods require quite a bit of overhead, either in the regex system
calls, or by using quite a few mallocs when going through the IN parser
operation.

We can cut down on this overhead by adding a "list" expression type,
denoted by the { } delimiters, where the previous layer definition becomes:

 LAYER
  ...
  CLASSITEM "type"
  CLASS
    EXPRESSION {primary,secondary,tertiary}
    ...
  END
END

The modifications to the code are fairly straightforward, and no backwards
incompatibilities are expected as this is a new functionality that does not
collide with previous mapserver expressions.
The changes can be seen here:
https://github.com/tbonfort/mapserver/commit/fae435d17b099485ab6792bfae3a7d8dd8992b4c.
(you can ignore the changes to mapparser.*, they correct a bug in the
IN
parser and aren't related to the list operator)

Our parser guru is OK with these changes, so I would like to vote to
include this new expression for our 6.4 release.

I'll start with my +1

best regards,
thomas
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