RFC 33: Removing msLayerWhichItems()...

[Daniel Morissette]

Steve Lime wrote:
> The performance concern is real although I don't know how big a deal it is for all drivers. I
> believe with shapefiles you get everything anyway.  Not sure about others. How would OGR
> be impacted?
> 

Your comment made me doubt so I ran both shapefiles and OGR data sources 
in the debugger and confirmed that when rendering a map, they both load 
only the attributes that are required.

In the shapefile case I believe you still need to do the file I/O to 
read the full attribute record from disk, but only the values of the 
required attributes are strdup'd and stored in the shape->values[] array.

OGR works in a similar way: the OGRFeature object returned by OGR 
contains all the attributes but we only copy the required attributes to 
the shape->values[] that is seen by MapServer.

I think the performance gain/loss would be more significant for 
shapefiles than for OGR data sources for this reason.

> msLayerWhichItems sets up the item indexes and you still need that for queries since CLASSes
> come into play.
> 

Plus, OGR uses special item indexes for some special OGR attributes that 
are extracted from the style strings (OGR:LabelText, OGR:LabelAngle, and 
OGR:LabelSize), so if we get rid of item indexes we'll need to think of 
another way to handle that.

> 
> One way to mitigate the performance impact might be to allow a user to explicitly set the ITEMS 
> array in the configuration file. I'm hoping that with PostGIS and Oracle Spatial that's essentially 
> what you do with the sub-selects. An added benefit of that would be that inline features could
> now have attributes:
> 
> LAYER
>   NAME 'anInlineLayer'
>   TYPE POLYGON
>   STATUS OFF
>   ITEMS name, type, area END
>   LABELITEM 'name'
>   FEATURE
>      POINTS ... END
>      VALUES 'St. Paul', 'CITY', '53045098.12' END
>   END
>   ...
> END
> 

I thought about providing the ability to specify the list of items in 
the layer as a possible solution as well, but then I realized that this 
was a move backwards for users since MapServer used to determine the 
list of items automatically, and from now on users would be required to 
do that manually for all layers if they care about performance.


I find it odd for me to be arguing *for* keeping msLayerWhichItems() 
since I never really understood nor liked it until today. In the 
interest of getting this done for 5.0 and if everybody wants to get rid 
of it then I won't object any more. I'll be happy with an optional 
ITEMS...END block for those who really care about that little bit of 
performance.

Daniel
-- 
Daniel Morissette
http://www.mapgears.com/