You're seeing an excentric edge case of the way "purely negative" "Boolean
Queries" work.
in this example...
description:ruby AND (-title:java)
you have a top level boolean query with two mandatory clauses. the second
clause in that top level query is a (bested) boolean query with a single
negative clause.
that nested boolean query with a single negative clause can never match
anything, and since it's a mandatory clause in the top level boolean
query, the entire query can never match anything.
In your second example...
description:ruby AND -title:java
...you have a top level boolean query with two clauses -- the first is
mandatory and the second is prohibited (the "AND" is ignored for the
second clause because the "-" overrides it -- see this link for reasons
why i hate AND/OR and you shoulddefinitely not try to mix them with +/-:
http://lucidworks.com/blog/why-not-and-or-and-not/ )
the end result is a top level query that will match anything matched by
the first clause, excluding things matched by the second clause.
In your remaining examples is where a small bit of esoteric solr behavior
comes into play...
(-title:java)
-title:java
...in both of these cases, you get a boolean query containing a single
prohibitied clause. in the first case the parents make that boolean query
explicitly, in the second case the use of "-" to prohibit something force
the parser to wrap it in a boolean query.
when solr sees that your top level query is a boolean query and *all* of
the clauses of that top level boolean query are prohibited, solr tries to
help "do what you mean" by re-writing that query to...
(+*:* -title:java)
..ie: it adds a mandatory match all docs query as an additional clause.
this "do what you mean" logic applies only to the top level boolean query,
it doesn't recurse into sub-queries -- both because it's kind of
infeasible to do that in an nested structure of arbitrary queries (some of
which might be custom plugins) but also because even in simple cases, it
might not be what you want -- if you were programatically building up
clauses of boolean queries from a form widget or something like that, your
goal might in fact be that one particular nested-query only contain
clauses $X & $Y. if $X & $Y are both prohibited, then it might
make sense to you that "($X $Y)" should match nothing, and thus "foo bar
($X $Y)" should only match foo & bar, if it was suppose to match other
stuff as well, the widget would have added some (positive) $Z to the
nested query.
-Hoss
http://www.lucidworks.com/
