[LUCENE-9093] Unified highlighter with word separator never gives context to the left

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 8.5
• Component/s: modules/highlighter
• Lucene Fields: New
• Assignee: David Smiley
• Reporter: Tim Retout
• Created: 20/Oct/17 11:49
• Updated: 27/Aug/24 15:23
• Resolved: 01/Jan/20 06:14

Description

When using the unified highlighter with hl.bs.type=WORD, I am not able to get context to the left of the matches returned; only words to the right of each match are shown. I see this behaviour on both Solr 6.4 and Solr 7.1.

Without context to the left of a match, the highlighted snippets are much less useful for understanding where the match appears in a document.

As an example, using the techproducts data with Solr 7.1, given a search for "apple", highlighting the "features" field:

http://localhost:8983/solr/techproducts/select?hl.fl=features&hl=on&q=apple&hl.bs.type=WORD&hl.fragsize=30&hl.method=unified

I see this snippet:

"<em>Apple</em> Lossless, H.264 video"

Note that "Apple" is anchored to the left. Compare with the original highlighter:

http://localhost:8983/solr/techproducts/select?hl.fl=features&hl=on&q=apple&hl.fragsize=30

And the match has context either side:

", Audible, <em>Apple</em> Lossless, H.264 video"

(To complicate this, in general I am not sure that the unified highlighter is respecting the hl.fragsize parameter, although SOLR-9935 suggests support was added. I included the hl.fragsize param in the unified URL too, but it's making no difference unless set to 0.)

Issue Links

• causes: SOLR-14628 hl.fragsizeIsMinimum default ought to be 'true' for performance
• links to: GitHub Pull Request #1123