tech-studies

Highlighting

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Highlighting

Highlighting returns snippets of matching fields with the query terms wrapped in tags, so a UI can show why a document matched.

Why it matters

Search results without highlighted context force users to guess relevance. Highlighting extracts the best-matching fragments and emphasizes the hit terms — standard in any search UI — but it re-analyzes or re-reads field content per hit, so the choice of highlighter directly affects latency and storage.

How it works

Three highlighter implementations trade speed for accuracy and storage.

HighlighterNeedsSpeedAccuracy on phrases
unified (default)nothing extragoodgood (BM25-ranked fragments)
plainre-analyzes _sourceslow on big fieldsexact, small docs only
fvh (fast vector)term_vector: with_positions_offsetsfast on big fieldsexcellent
  • unified uses the Lucene Highlighter with a BM25-like fragment scorer; works from postings or term vectors when available.
  • Tags — default <em>…</em>; override with pre_tags/post_tags.
  • Fragmentation — fragment_size (chars, default 100) and number_of_fragments (default 5) bound the snippets; number_of_fragments:0 returns the whole field highlighted.
  • require_field_match:false highlights fields other than the one queried (useful with multi_match).

Example

{ "query": { "match": { "body": "climate change" } },
  "highlight": {
    "fields": { "body": { "fragment_size": 120, "number_of_fragments": 3 } },
    "pre_tags": ["<mark>"], "post_tags": ["</mark>"] } }
// → "...the science of <mark>climate</mark> <mark>change</mark> shows..."

The unified highlighter ranks the 3 best fragments by relevance, not document order.

Pitfalls

  • fvh needs term vectors — enabling term_vector: with_positions_offsets roughly doubles that field’s index size; only worth it for large highlighted fields.
  • plain on big fields — re-analyzes the whole _source per hit; can dominate query latency on multi-KB documents.
  • Mapping mismatch — highlighting a field whose analyzer changed since indexing produces wrong offsets/missing marks.
  • number_of_fragments:0 cost — returning the entire field highlighted defeats fragment limits and inflates response size.

See also

Graph View

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