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From "Rose, Stuart J" <>
Subject RE: Only highlight terms that caused a search hit/match
Date Sun, 16 Feb 2014 18:35:43 GMT
Hi Steve, 

We leveraged the SpanQuery and Highlighting APIs in 3.5 a couple of years ago to do this.
In order to get accurate doc hits for the types of phrases that we needed to support search
on, we defined a phrase query syntax and then implemented a span query parser to create a
nested structure of span operations that embody the query. 

The test output below gives the span structure that we generate and then the resulting highlights
for each query. 

	spanOr([text:a, spanNear([text:b, text:z], 987654321, false)])
	<B>a</B> b c

	spanNear([spanNear([text:x, text:y, text:z], 0, true), text:a], 10, false)
	y z <B>x</B> <B>y</B> <B>z</B> <B>a</B>

I'll check to see if we can make it available as a starting point for what Mike is suggesting.

In the meantime, I recommend verifying that each span query is created as intended, keeping
in mind that doc hits may be 'valid', but might have matched for the wrong reason and therefore
have mismatched highlighting. 


-----Original Message-----
From: Michael McCandless [] 
Sent: Saturday, February 15, 2014 2:54 AM
To: Lucene Users
Subject: Re: Only highlight terms that caused a search hit/match

Unfortunately, all Lucene's highlighters are "approximate" in this
regard: there is no guarantee that the shown snippets, if they were a single little document,
would have matched the query.

Even the newest highlighter, PostingsHighlighter, doesn't look at positions, e.g. a PhraseQuery
highlight could be "wrong", though "typically" the snippets with all terms from the phrase
will scorer higher and be more likely to be picked in practice.

Net/net I think a "precise highlighter", would be a nice addition to Lucene, but it is a challenge
because you need to turn every leaf query into a positional query, even queries like TermQuery
that normally don't touch positions, and then you need to follow the query tree while you
highlight so that in your first example a OR (b AND z), having picked a snippet or two for
a, you then also go and pick a snippet or two for the b AND z clause, and then present them
both together.

It's a hard problem but it would make a great addition.

Mike McCandless

On Fri, Feb 14, 2014 at 7:05 PM, Steve Davids <> wrote:
> Hello,
> I have recently been given a requirement to improve document highlights within our system.
Unfortunately, the current functionality gives more of a best-guess on what terms to highlight
vs the actual terms to highlight that actually did perform the match. A couple examples of
issues that were found:
> Nested boolean clause with a term that doesn't exist ANDed with a term 
> that does highlights the ignored term in the query
> Text: a b c
> Logical Query: a OR (b AND z)
> Result: <b>a</b> <b>b</b> c
> Expected: <b>a</b> b c
> Nested span query doesn't maintain the proper positions and offsets
> Text: y z x y z a
> Logical Query: ("x y z", a) span near 10
> Result: <b>y</b> <b>z</b> <b>x</b> <b>y</b>
<b>z</b> <b>a</b>
> Expected: y z <b>x</b> <b>y</b> <b>z</b> <b>a</b>
> I am currently using the Highlighter with a QueryScorer and a SimpleSpanFragmenter. While
looking through the code it looks like the entire query structure is dropped in the WeightedSpanTermExtractor
by just grabbing any positive TermQuery and flattening them all into a simple Map which is
then passed on to highlight all of those terms. I believe this over simplification of term
extraction is the crux of the issue and needs to be modified in order to produce more "exact"
> I was brainstorming with a colleague and thought perhaps we can spin up a MemoryIndex
to index that one document and start performing a depth-first search of all queries within
the overall Lucene query graph. At that point we can start querying the MemoryIndex for leaf
queries and start walking back up the tree, pruning branches that don't result in a search
hit which results in a map of actual matched query terms. This approach seems pretty painful
but will hopefully produce better matches. I would like to see what the experts on the mailing
list would have to say about this approach or is there a better way to retrieve the query
terms & positions that produced the match? Or perhaps there is a different Highlighter
implementation that should be used, though our user queries are extremely complex with a lot
of nested queries of various types.
> Thanks,
> -Steve

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