Hi,
looking at join support in WFS 2.0 I _believe_ to have discovered a
limitation
in the way joins are implemented by looking at a 3 way join and then
generalizing the issue to a N way join.
Basically, the way joining has been implemented allows to join a central
feature type FT1
to any other feature type, but you cannot create a chain of joins that does
not have a central element.
In other terms, it's possible to do a "star join" with a central element
joined to
everything else:
(FT1 join FT2) and (FT1 join FT3) and (FT1 join FT3) and ... (FT1 join FTN)
but not a "chain join" with no central element:
(FT1 join FT2) and (FT2 join FT3) and (FT3 join FT4) and ... (FTN-1 join
FTN)
Now, limited to the 3 way case, it's possible to do a rewrite to create the
central element,
from:
(FT1 join FT2) and (FT2 join FT3)
to:
(FT2 join FT1) and (FT2 join FT3)
thus respecting the "central feature type" structure.
Buf the moment we go to a 4 way join, the thing falls apart.
The way I understand things, to make a "chain join" possible, we'd need to
allow
a Join to have sub-joins just like Query, and the particular case of
chain join would be represented as:
Query(join1(join2(join3(....(JoinN))))
Any feedback?
Do you have a feel of whether that would require major changes to the JDBC
store?
Cheers
Andrea
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