Constant score query in Elasticsearch - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constant score query in Elasticsearch?

A constant score query wraps another query and returns a fixed score for all matching documents, ignoring the relevance score of the wrapped query.

beginner

Why would you use a constant score query instead of a regular query?

To improve performance when you only need to filter documents without scoring them differently, or when you want all matching documents to have the same score.

beginner

How do you specify a constant score query in Elasticsearch JSON DSL?

You use the constant_score object with a filter or query inside it, like this:

{"constant_score": {"filter": { ... }}}

beginner

What happens to the score of documents matched by a constant score query?

All matched documents receive the same fixed score, which is 1 by default, regardless of how well they match the inner query.

intermediate

Can you combine a constant score query with other queries in Elasticsearch?

Yes, you can combine it inside a bool query or other compound queries to control scoring behavior for parts of your search.

What does a constant score query do in Elasticsearch?

A Sorts documents by date

B Returns the same score for all matching documents

C Scores documents based on term frequency

D Filters documents without scoring

A constant score query returns a fixed score for all documents that match the inner query or filter.

Which key is used inside a constant score query to specify the condition?

A match

B range

C term

D filter

The filter key is used inside a constant score query to specify the condition that documents must meet.

What is the default score assigned by a constant score query to matching documents?

A 1

B 0

C Variable based on relevance

D 10

By default, all matching documents get a score of 1 in a constant score query.

Why might you choose a constant score query over a regular query?

A To highlight matched terms

B To get more accurate relevance scores

C To improve performance by skipping scoring calculations

D To sort results by score

Constant score queries improve performance by avoiding complex scoring calculations.

Can a constant score query be used inside a bool query?

A Yes

B No

C Only in filters

D Only with term queries

Constant score queries can be combined inside bool queries to control scoring behavior.

Explain what a constant score query is and when you would use it.

Think about scoring and filtering in search results.

Describe how to write a constant score query in Elasticsearch JSON DSL.

Focus on the JSON keys and nesting.

Practice

1. What is the main purpose of a constant_score query in Elasticsearch?

easy

A. To filter documents and assign the same score to all matched documents

B. To sort documents by their relevance score

C. To perform a full-text search with varying scores

D. To aggregate documents by a field value

Solution

1. Step 1: Understand the role of constant_score query

   The constant_score query filters documents and assigns the same score to all matches, ignoring relevance scoring.

2. Step 2: Compare with other query types

   Unlike full-text queries that score by relevance, constant_score keeps scores equal for filtered documents.

3. Final Answer:

   To filter documents and assign the same score to all matched documents -> Option A

4. Quick Check:

   constant_score = equal scoring for filtered docs [OK]

Hint: Constant score means all matched docs get the same score [OK]

Common Mistakes:

• Thinking it sorts by relevance
• Confusing with full-text scoring
• Assuming it aggregates data

2. Which of the following is the correct syntax to create a constant score query with a filter on field status equal to active?

easy

A. {"constant_score": {"filter": {"match": {"status": "active"}}}}

B. {"constant_score": {"query": {"term": {"status": "active"}}}}

C. {"constant_score": {"term": {"status": "active"}}}

D. {"constant_score": {"filter": {"term": {"status": "active"}}}}

Solution

1. Step 1: Identify correct key for filtering

   The constant_score query uses the key "filter" to specify the filter clause, not "query" or direct term.

2. Step 2: Check filter syntax

   The filter should be a term query inside the filter object, correctly written as {"term": {"status": "active"}}.

3. Final Answer:

   {"constant_score": {"filter": {"term": {"status": "active"}}}} -> Option D

4. Quick Check:

   Use "filter" with term inside constant_score [OK]

Hint: Use "filter" key with term query inside constant_score [OK]

Common Mistakes:

• Using "query" instead of "filter"
• Placing term query directly under constant_score
• Using match instead of term for exact filter

3. Given this Elasticsearch query snippet, what will be the score of each matched document?

{
  "constant_score": {
    "filter": { "term": { "category": "books" } },
    "boost": 3
  }
}

medium

A. The score will vary based on relevance

B. The score will be 3 for all matched documents

C. The score will be 1 for all matched documents

D. The score will be 0 because boost is ignored

Solution

1. Step 1: Understand constant_score with boost

   The constant_score query assigns the same score to all matched documents, and the boost multiplies that score.

2. Step 2: Calculate the score

   By default, constant_score gives score 1, multiplied by boost 3, resulting in score 3 for all matches.

3. Final Answer:

   The score will be 3 for all matched documents -> Option B

4. Quick Check:

   constant_score x boost = 3 [OK]

Hint: Boost multiplies constant score, so score = boost value [OK]

Common Mistakes:

• Assuming scores vary by relevance
• Ignoring the boost value
• Thinking boost is ignored in constant_score

4. Identify the error in this constant score query:

{
  "constant_score": {
    "filter": {
      "terms": {
        "tags": ["tech", "science"]
      }
    },
    "boost": "2"
  }
}

medium

A. The boost value should be a number, not a string

B. The filter key should be replaced with query

C. The terms query cannot be used inside constant_score

D. The tags field should be inside a match query

Solution

1. Step 1: Check boost data type

   The boost parameter must be a numeric value, not a string. Here, "2" is a string, which is invalid.

2. Step 2: Validate filter and terms usage

   Using "filter" with a "terms" query is valid syntax, so no error there.

3. Final Answer:

   The boost value should be a number, not a string -> Option A

4. Quick Check:

   boost must be numeric, not string [OK]

Hint: Boost must be a number, not quoted string [OK]

Common Mistakes:

• Using string for boost instead of number
• Replacing filter with query incorrectly
• Misunderstanding terms query usage

5. You want to find all documents where status is published and boost their score by 5, but keep all matched documents scored equally. Which query correctly achieves this?

hard

A. { "match": { "status": "published", "boost": 5 } }

B. { "bool": { "must": { "term": { "status": "published" } }, "boost": 5 } }

C. { "constant_score": { "filter": { "term": { "status": "published" } }, "boost": 5 } }

D. { "constant_score": { "query": { "term": { "status": "published" } }, "boost": 5 } }

Solution

1. Step 1: Use constant_score with filter and boost

   To keep scores equal and apply a boost, use constant_score with a filter clause and boost parameter.

2. Step 2: Validate correct syntax

   { "constant_score": { "filter": { "term": { "status": "published" } }, "boost": 5 } } uses "filter" with a term query and numeric boost, which is the correct syntax.

3. Step 3: Check other options

   { "bool": { "must": { "term": { "status": "published" } }, "boost": 5 } } uses bool query which scores by relevance, not constant. { "match": { "status": "published", "boost": 5 } } is a match query with boost but scores vary. { "constant_score": { "query": { "term": { "status": "published" } }, "boost": 5 } } incorrectly uses "query" instead of "filter" inside constant_score.

4. Final Answer:

   { "constant_score": { "filter": { "term": { "status": "published" } }, "boost": 5 } } -> Option C

5. Quick Check:

   constant_score + filter + boost = equal boosted scores [OK]

Hint: Use constant_score with filter and numeric boost for equal boosted scores [OK]

Common Mistakes:

• Using query instead of filter inside constant_score
• Using bool or match queries that score variably
• Setting boost as string instead of number