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https://issues.apache.org/jira/browse/LUCENE-6570?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14592755#comment-14592755
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Hoss Man commented on LUCENE-6570:
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bq. Are you suggesting that this new builder should be backported to 5.x and
that the setters on BooleanQuery should be marked as deprecated?
yes.
bq. I did not consider this option because it would make BooleanQuery have the
API of an immutable query while nothing would guarantee that it cannot be
modified since we would need to keep the old deprecated setters. So I just made
it a breaking change for 6.0.
I think it would be good to get the API out there for 5.x users, so they can
start using it. Even if the underlying BooleanQuery objects themselves are not
immutable, encouraging people to use the Builder pattern to create them, and
discouraging them from _expecting_ to be able to mutate the BooleanQuery
objects seems like a good idea to get out there as early as possible.
bq. I am willing to document the fact that we clone sub queries, but I am on
the fence about removing it, since without defensively cloning, the
BooleanQuery would still be mutable while this issue is about making sure it
cannot change.
There's a difference between saying the BooleanQuery is immutable and cannot
change and saying the queries wrapped by the BooleanQuery are cloned & no
longer accessible and cannot cahnge -- that's something that wasn't clear from
your issue description until I looked closer at your commit.
By comparison, "Collections.singletonList(T o)" is documented to return an
immutable list, but it doesn't clone 'o' -- it doesn't try to prevent you from
calling o.changeState() after you construct that list.
bq. For instance, consider the following sequence of operations: ...
The change in run time behavior of code like that is exactly why this change
scares me -- it's very similar to what users may have come to expect from code
like my example if they want to do something like call
REUSED_FILTER.setBost(foo) to dynamically tweak relevancy of many queries (all
at once) at runtime.
bq. One motivation behind this change is to be able to enable the query cache
by default in IndexSearcher (currently off), which we can only do if queries
can reliably be used as cache keys.
I can appreciate that goal, but i don't think it's ever going to be feasible to
turn that on by default in the truly generic case of _any_ arbitrary lucene
application, where people might have custom Query impls.
Bottom line: i think there is a decent risk that this under the covers, no way
to turn off, cloning of sub-queries will have some serious negative
consequences for some use cases, but will really only help _if_ you use a query
cache and _if_ you get a high hit rate on that cache and _if_ your code is
written weirdly/broken to call setBoost on queries after you use them -- which
doesn't make sense to do at all if you are using a query cache.
Consider again that REUSED_FILTER example i mentioned in my last comment --
assuming the application is "well behaved", and doesn't call setBoost at add
times: even w/o the implicit clone in BooleanQuery it should work great with a
query cache enabled, and would use a lot less ram then with the implicit
sub-query cloning in the BooleanQuery.Builder.
But if an application _does_ start trying to keep refrences to previously
constructed Query instances, and call mutating methods (like setBoost) at
runtime, then really they aren't going to be able to safely use the query cache
at all -- regardless of whether you have this implicit clone in BooleanQuery's
builder.
> Make BooleanQuery immutable
> ---------------------------
>
> Key: LUCENE-6570
> URL: https://issues.apache.org/jira/browse/LUCENE-6570
> Project: Lucene - Core
> Issue Type: Task
> Reporter: Adrien Grand
> Assignee: Adrien Grand
> Priority: Minor
> Fix For: 6.0
>
> Attachments: LUCENE-6570.patch
>
>
> In the same spirit as LUCENE-6531 for the PhraseQuery, we should make
> BooleanQuery immutable.
> The plan is the following:
> - create BooleanQuery.Builder with the same setters as BooleanQuery today
> (except setBoost) and a build() method that returns a BooleanQuery
> - remove setters from BooleanQuery (except setBoost)
> I would also like to add some static utility methods for common use-cases of
> this query, for instance:
> - static BooleanQuery disjunction(Query... queries) to create a disjunction
> - static BooleanQuery conjunction(Query... queries) to create a conjunction
> - static BooleanQuery filtered(Query query, Query... filters) to create a
> filtered query
> Hopefully this will help keep tests not too verbose, and the latter will also
> help with the FilteredQuery derecation/removal.
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