Highlighting matched text in Elasticsearch - Step-by-Step Execution

Concept Flow - Highlighting matched text

Send Search Query with Highlight

↓

Elasticsearch Matches Documents

↓

Highlight Matched Text in Fields

↓

Return Results with Highlighted Snippets

↓

Client Displays Highlighted Text

The search query is sent with highlight instructions, Elasticsearch finds matches, highlights matched text, and returns results with highlighted snippets.

Execution Sample

Elasticsearch

{
  "query": {
    "match": {"content": "apple"}
  },
  "highlight": {
    "fields": {"content": {}}
  }
}

This query searches for 'apple' in the 'content' field and highlights matched text in the results.

Execution Table

Step	Action	Query Part	Result/Effect
1	Send search query	{"match":{"content":"apple"},"highlight":{"fields":{"content":{}}}}	Query received by Elasticsearch
2	Match documents	match content: apple	Documents containing 'apple' found
3	Highlight matched text	highlight fields: content	Matched 'apple' text wrapped with highlight tags
4	Return results	results with highlight	Results include original fields plus highlighted snippets
5	Client displays	highlighted snippets	User sees 'apple' highlighted in search results

💡 All matching documents processed and highlighted text returned to client

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
query	{}	{"match":{"content":"apple"}}	{"match":{"content":"apple"},"highlight":{"fields":{"content":{}}}}	Same query throughout
matched_docs	[]	[doc1, doc2]	[doc1, doc2]	[doc1, doc2] with highlights
highlighted_text	N/A	N/A	["<em>apple</em>"]	["<em>apple</em>"]

Key Moments - 3 Insights

Why does the highlight only appear in the 'content' field?

Because the highlight section specifies only the 'content' field for highlighting, as shown in execution_table step 3.

What happens if no documents contain the word 'apple'?

No documents match in step 2, so no highlights are created or returned, ending the process early.

Does the original text change when highlighted?

No, the original text stays the same; highlight adds tags around matched words in the returned snippet only (step 3).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Elasticsearch find documents containing 'apple'?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Action' and 'Result/Effect' columns in execution_table row for Step 2

According to variable_tracker, what is the value of 'highlighted_text' after Step 3?

A []

B ["apple"]

C ["<em>apple</em>"]

D N/A

💡 Hint

Look at the 'highlighted_text' row under 'After Step 3' in variable_tracker

If the highlight field was changed to 'title' instead of 'content', what would happen?

A Highlighting would appear in the 'title' field instead of 'content'

B Highlighting would appear in both 'title' and 'content'

C No highlighting would appear

D Query would fail

💡 Hint

Refer to execution_table step 3 where highlight fields are specified

Concept Snapshot

Highlighting matched text in Elasticsearch:
- Add 'highlight' section in query
- Specify fields to highlight
- Elasticsearch wraps matched terms with tags
- Results include highlighted snippets
- Client shows highlighted text to user

Full Transcript

This visual execution shows how Elasticsearch highlights matched text. First, a search query with a highlight section is sent. Elasticsearch finds documents matching the query term 'apple' in the 'content' field. Then it wraps the matched text with highlight tags like <em>. The results returned include these highlighted snippets. Finally, the client displays the highlighted text to the user. Variables like matched_docs and highlighted_text change as the query runs. Key points include that only specified fields are highlighted and original text is unchanged. The quiz checks understanding of steps and variable states.

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