On 16/02/12 02:04, Holger Knublauch wrote:
I have a rather advanced question and there may not be a general
solution, but I'll try anyway...
Problem: Given an arbitrary SPARQL WHERE clause, I want to insert a
line
?this a ?SOME_TYPE .
so that it always gets executed "first" - I want ?this to be bound to
all instances of a given class before the rest of the WHERE clause
executes. For example if I have
The optimizer will only rearrange things when it does not change the
answers.
Putting it first, just after the { should be OK but the algebra is more
robust.
BIND operates (like FILTER) after the patterns matched in that
particular block so anywhere in the block will work.
WHERE {
{
BIND (ex:fun(?this) AS ?result) .
}
FILTER (?result != 1)
}
which is:
(filter (!= ?result 1)
(extend ((?result (ex:fun ?this)))
(table unit)))
e.g. filter of
bind of
pattern match
(table unit) being the empty basic graph pattern
then I want to programmatically rewrite this to
WHERE {
{
?this a ?SOME_TYPE .
BIND (ex:fun(?this) AS ?result) .
}
FILTER (?result != 1)
}
That is, you want:
(filter (!= ?result 1)
(extend ((?result (ex:fun ?this)))
(bgp (triple ?this rdf:type ?SOME_TYPE))))
Is there any clean way of implementing this? My naive approach would
be to walk the open parentheses until I find the first one with a
matching closing parenthesis (or do the equivalent operation on the
Syntax tree instead of strings. Given the policy that SPARQL is
executed from the inside out, is this a safe assumption? Are there
alternative approaches to achieving the goal (of iterating over all
instances of a class)? Are there cases where I need to repeat the
binding of ?this in multiple sub-blocks? Or will it be more efficient
to do the iteration "outside" and simply ask the same query for each
instance, with ?this pre-bound with initial bindings?
Another suggestion: put a marker pattern into the query and then you can
find it again.
Put in the triple
<tq:holger> <tq:holger> <tq:holger> .
and then you can go and find it again so you know where it is after
parsing and algebra generation.
(filter (!= ?result 1)
(extend ((?result (ex:fun ?this)))
(bgp (triple <tq:holger> <tq:holger> <tq:holger>))))
Andy
