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Re: [SML] Whether to support Attribute or not?

  • From: Pavel Velikhov <pvelikho@c...>
  • To: "Clark C. Evans" <clark.evans@m...>
  • Date: Mon, 29 Nov 1999 15:04:14 -0800

Re: [SML] Whether to support Attribute or not?

"Clark C. Evans" wrote:
> On Mon, 29 Nov 1999, Pavel Velikhov wrote:
> ? ? ? ?Thing?
> ? ? ?  ?Id?foo?/Id?
> ? ? ? ?/Thing?
> ? ? ?
> ? ? ? But, well.... yuck.  This is just... strange.
> ? ? ?
> ? ? ? Doesn't it strike you as perverse to dive into the sub-elements of an
> ? ? ? element to get a handle to the parent?
> ? ?
> ? ? Yes it does.  It also requires that the _entire_ element be read
> ? ? in (till its end tag is encountered) just in the case that a child
> ? ? provides details which modify the parent -- in this case, the
> ? ? parent's identifier.
> ? ?
> ?
> ? But every child provides details that modify the parent.
> ? Consider this element:
> ?
> ? ?home?
> ?       ?zip?91111?/zip?
> ? ?/home?
> ?
> ? And then the following element:
> ?
> ? ?home?
> ?       ?local_schools?
> ?               ?school?S1?/school?
> ?               ?school?S2?/school?
> ?               ...
> ?       ?/local_schools?
> ? ?/home?
> ?
> ? Same problem here, right? Every child modifies the parent.
> ? Should not these cases be all semantically equivalent?
> If you have to "dig" into the contents to figure
> out which "processing pipeline" the content is
> placed on then you have effectively elminated
> all hope of sequential processing.  Thus, you
> need random access to the information stream.
> And hence, "large" memory requiremens, thus
> undermining the apparant "simplicity" of SML.

Well, I don't agree with you on this point. I have been working
on a streaming query processor for XML for some time, and the
problem is just not there. Why is it a problem going into the
element one or two levels deep? Besides the chances that you will
encode all the information relevant to the query processor into
attributes is very small. Consider the example above with the
local schools: what if I want to select all homes that have
some specific school in their neighborhood? This information
cannot be encoded in the attributes, since its nested. But I
can still stream such queries, all I have to do is descend one
level into each element and the find the school that satisfies
the condition.

Pavel Velikhov

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