Highlighting matched text in Elasticsearch - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the highlighted output for a simple match query?

Given the following Elasticsearch query with highlighting, what will be the highlighted fragment in the search results?

Elasticsearch

{
  "query": {
    "match": {
      "content": "quick brown"
    }
  },
  "highlight": {
    "fields": {
      "content": {}
    }
  }
}

A ["The <em>quick</em> <em>brown</em> fox jumps"]

B ["The quick brown fox jumps"]

C ["The <strong>quick</strong> <strong>brown</strong> fox jumps"]

D ["The <highlight>quick</highlight> <highlight>brown</highlight> fox jumps"]

💡 Hint

Elasticsearch uses tags by default for highlighting matched terms.

By default, Elasticsearch wraps matched terms in tags in the highlighted fragments.

🧠 Conceptual

intermediate

Which option correctly changes the highlight tags?

You want to change the default tags to tags in Elasticsearch highlighting. Which highlight configuration achieves this?

A "highlight": { "fields": { "content": {} }, "tags": ["<strong>", "</strong>"] }

B "highlight": { "fields": { "content": {} }, "highlight_tags": ["<strong>", "</strong>"] }

C "highlight": { "fields": { "content": {} }, "pre_tags": ["<strong>"], "post_tags": ["</strong>"] }

D "highlight": { "fields": { "content": {} }, "wrap_tags": ["<strong>", "</strong>"] }

💡 Hint

Look for the official parameters to customize highlight tags.

Elasticsearch uses pre_tags and post_tags to customize the tags wrapping matched terms.

🔧 Debug

advanced

Why does this highlight query return no highlights?

Consider this Elasticsearch query. Why does it return no highlighted fragments even though the document contains the word 'fox'?

Elasticsearch

{
  "query": {
    "match": {
      "content": "fox"
    }
  },
  "highlight": {
    "fields": {
      "content": { "type": "fvh" }
    }
  }
}

A The field 'content' is not stored or indexed with term vectors required by 'fvh' highlighter.

B The 'fvh' highlighter does not support the 'match' query type.

C The highlight field name 'content' is misspelled.

D The query syntax is invalid causing no results.

💡 Hint

The 'fvh' highlighter requires specific field settings.

The 'fvh' (Fast Vector Highlighter) requires the field to be indexed with term vectors enabled. Without this, no highlights are returned.

📝 Syntax

advanced

Identify the syntax error in this highlight query

Which option contains a syntax error that will cause Elasticsearch to reject the highlight query?

Elasticsearch

{
  "query": {
    "match": {
      "content": "dog"
    }
  },
  "highlight": {
    "fields": {
      "content": {
        "pre_tags": ["<em>"],
        "post_tags": ["</em>"]
      }
    }
  }
}

A "highlight": { "fields": { "content": { "pre_tags": ["<em>"], "post_tags": ["</em>"] } } } }

B { "highlight": { "fields": { "content": { "pre_tags": ["<em>"], "post_tags": ["</em>"] } } } }

C } } } } ]">me/<"[ :"sgat_tsop" ,]">me<"[ :"sgat_erp" { :"tnetnoc" { :"sdleif" { :"thgilhgih" {

D { "highlight": { "fields": { "content": { "pre_tags": "<em>", "post_tags": "</em>" } } } }

💡 Hint

Check the data types of pre_tags and post_tags values.

pre_tags and post_tags must be arrays of strings, not single strings.

🚀 Application

expert

How many highlighted fragments will be returned?

Given this query with highlight settings, how many fragments will Elasticsearch return if the content contains the phrase 'lazy dog' repeated 3 times?

Elasticsearch

{
  "query": {
    "match_phrase": {
      "content": "lazy dog"
    }
  },
  "highlight": {
    "fields": {
      "content": {
        "number_of_fragments": 2,
        "fragment_size": 10
      }
    }
  }
}

A 0

B 2

C 1

D 3

💡 Hint

The number_of_fragments setting limits how many fragments are returned.

Even if the phrase appears 3 times, the highlight setting number_of_fragments: 2 limits the output to 2 fragments.

Practice

1. What is the main purpose of the highlight section in an Elasticsearch query?

easy

A. To filter documents before searching

B. To sort the search results by relevance

C. To update documents in the index

D. To show matched words clearly in search results

Solution

1. Step 1: Understand the role of highlighting

   Highlighting is used to emphasize matched words in search results so users can see why a document matched.

2. Step 2: Compare with other query parts

   Sorting, filtering, and updating are different functions not related to highlighting matched text.

3. Final Answer:

   To show matched words clearly in search results -> Option D

4. Quick Check:

   Highlight = show matched words [OK]

Hint: Highlight section shows matched words in results [OK]

Common Mistakes:

• Confusing highlight with sorting or filtering
• Thinking highlight updates documents
• Assuming highlight changes search relevance

2. Which of the following is the correct syntax to add highlighting for the field content in an Elasticsearch query?

easy

A. "highlight": {"fields": {"content": {}}}

B. "highlight": {"field": "content"}

C. "highlight": {"content": {"fields": {}}}

D. "highlight": {"fields": ["content"]}

Solution

1. Step 1: Recall correct highlight syntax

   The correct syntax uses "highlight" with "fields" as an object mapping field names to options, like {"content": {}}.

2. Step 2: Check each option

   "highlight": {"fields": {"content": {}}} matches the correct structure. Options B, C, and D use incorrect keys or data types.

3. Final Answer:

   "highlight": {"fields": {"content": {}}} -> Option A

4. Quick Check:

   Highlight fields syntax = "highlight": {"fields": {"content": {}}} [OK]

Hint: Use "highlight": {"fields": {"fieldname": {}}} [OK]

Common Mistakes:

• Using "field" instead of "fields"
• Putting fields inside an array instead of object
• Misplacing keys inside highlight

3. Given this Elasticsearch query snippet, what will the highlight section return for a match in the title field?

{
  "query": {"match": {"title": "quick fox"}},
  "highlight": {"fields": {"title": {}}}
}

medium

A. An error because highlight requires custom tags

B. Only the word "quick" highlighted, "fox" ignored

C. The matched words "quick" and "fox" wrapped in default highlight tags

D. No highlighting because tags are missing

Solution

1. Step 1: Understand default highlight behavior

   When highlight is enabled with empty options, Elasticsearch wraps matched words with default tags like <em> and </em>.

2. Step 2: Analyze the query

   The query matches both "quick" and "fox" in the title, so both words will be highlighted.

3. Final Answer:

   The matched words "quick" and "fox" wrapped in default highlight tags -> Option C

4. Quick Check:

   Default highlight tags wrap matched words [OK]

Hint: Empty highlight options use default tags to wrap matches [OK]

Common Mistakes:

• Thinking highlight needs custom tags to work
• Assuming only first matched word is highlighted
• Believing missing tags cause errors

4. Identify the error in this Elasticsearch highlight query snippet:

{
  "query": {"match": {"description": "fast car"}},
  "highlight": {"fields": ["description"]}
}

medium

A. "match" query is invalid for highlighting

B. "fields" should be an object, not an array

C. "highlight" must be inside "query"

D. "description" field cannot be highlighted

Solution

1. Step 1: Check highlight fields syntax

   The "fields" key must be an object mapping field names to options, not an array.

2. Step 2: Verify other parts

   "match" query is valid, "highlight" is correctly placed, and any field can be highlighted if indexed.

3. Final Answer:

   "fields" should be an object, not an array -> Option B

4. Quick Check:

   Highlight fields = object, not array [OK]

Hint: Highlight fields must be an object, not a list [OK]

Common Mistakes:

• Using array instead of object for fields
• Placing highlight inside query incorrectly
• Assuming some fields can't be highlighted

5. You want to highlight matched text in both title and content fields with custom tags <strong> and </strong>. Which is the correct highlight section to achieve this?

hard

A. "highlight": {"fields": {"title": {}, "content": {}}, "pre_tags": [""], "post_tags": [""]}

B. "highlight": {"fields": ["title", "content"], "pre_tags": "", "post_tags": ""}

C. "highlight": {"fields": {"title": {"tags": ["", ""]}, "content": {"tags": ["", ""]}}}

D. "highlight": {"fields": {"title": {}, "content": {}}, "tags": ["", ""]}

Solution

1. Step 1: Understand how to set custom tags

   Custom tags are set globally using "pre_tags" and "post_tags" arrays outside the fields object.

2. Step 2: Check each option's structure

   "highlight": {"fields": {"title": {}, "content": {}}, "pre_tags": [""], "post_tags": [""]} correctly sets "fields" as an object and uses "pre_tags" and "post_tags" arrays. Options B, C, and D misuse keys or data types.

3. Final Answer:

   "highlight": {"fields": {"title": {}, "content": {}}, "pre_tags": [""], "post_tags": [""]} -> Option A

4. Quick Check:

   Custom tags use pre_tags/post_tags arrays [OK]

Hint: Use pre_tags/post_tags arrays for custom highlight tags [OK]

Common Mistakes:

• Using arrays for fields instead of objects
• Setting tags inside each field incorrectly
• Using single strings instead of arrays for tags