[mapguide-internals] PATCH: GwsQueryEngine chooses incorrect join algorithm for joins involving SDF/SHP sources

[Jackie Ng]

Hi All,

I have attached a patch for ticket #1888 (GwsQueryEngine evaluates incorrect
join algorithm for joins involving SDF/SHP sources)

http://trac.osgeo.org/mapguide/ticket/1888

Simply put, the reason Feature Joins can perform really terrible for joins
involving SDF or SHP files is because sortability is a critical factor in
join algorithm selection, and under the existing code the SDF and SHP
providers both fail the sortability check, which gives us the worst choices
in join algorithms. 

This check is wrong because the two providers can actually output sorted
query results via the FdoIExtendedSelect command, but that command only
allows sorting on one property.

So the sortability check has been updated in this patch. A side of the join
is now considered sortable if:

 1. It supports standard select ordering -OR-
 2. It supports the FdoIExtendedSelect command (or
SdfIExtendedSelect/ShpIExtendedSelect) and we're only joining on one
property.

The existing code already takes advantage of extended select w/ ordering if
available so nothing needs to be done there.

Please review. Thanks

- Jackie

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