Dis max query in Elasticsearch

Introduction

The Dis max query helps find the best matching documents by choosing the highest score from multiple queries. It is useful when you want to combine several search options but only care about the best match.

When searching for a word that might appear in different fields and you want the best match from any field.

When combining different search queries but want to avoid adding their scores together.

When you want to boost the score of the best matching query without mixing scores from others.

Syntax

Elasticsearch

{
  "dis_max": {
    "queries": [
      { "match": { "field1": "search text" } },
      { "match": { "field2": "search text" } }
    ],
    "tie_breaker": 0.7,
    "boost": 1.2
  }
}

queries is a list of queries to compare.

tie_breaker adds a small score from other queries to help when multiple queries match.

Examples

Basic dis max query searching 'apple' in title or description fields.

Elasticsearch

{
  "dis_max": {
    "queries": [
      { "match": { "title": "apple" } },
      { "match": { "description": "apple" } }
    ]
  }
}

Dis max query with a tie_breaker to add some score from other matches.

Elasticsearch

{
  "dis_max": {
    "queries": [
      { "match": { "title": "apple" } },
      { "match": { "description": "apple" } }
    ],
    "tie_breaker": 0.3
  }
}

Dis max query with a boost to increase the overall score of the best match.

Elasticsearch

{
  "dis_max": {
    "queries": [
      { "match": { "title": "apple" } },
      { "match": { "description": "apple" } }
    ],
    "boost": 2.0
  }
}

Sample Program

This query searches for the word 'coffee' in either the title or description fields. It picks the best score from these two matches and adds half of the other scores to help when both fields match. The whole score is then boosted by 1.1 times.

Elasticsearch

{
  "query": {
    "dis_max": {
      "queries": [
        { "match": { "title": "coffee" } },
        { "match": { "description": "coffee" } }
      ],
      "tie_breaker": 0.5,
      "boost": 1.1
    }
  }
}

Important Notes

The tie_breaker value is usually between 0 and 1. It controls how much the scores from non-best queries affect the final score.

Boost increases the importance of the whole dis max query compared to other queries.

Summary

Dis max query picks the highest score from multiple queries to find the best match.

Use tie_breaker to add some influence from other matching queries.

Boost can increase the overall importance of the dis max query.

Practice

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

easy

A. To select the highest score from multiple queries to find the best match

B. To combine all scores from multiple queries by summing them

C. To filter documents based on a single exact match

D. To sort documents alphabetically by a field

Solution

1. Step 1: Understand the dis_max query behavior

   The dis_max query evaluates multiple queries and picks the highest score among them for each document.

2. Step 2: Compare with other scoring methods

   Unlike sum or filter queries, dis_max focuses on the best single match score, not combined or filtered results.

3. Final Answer:

   To select the highest score from multiple queries to find the best match -> Option A

4. Quick Check:

   dis_max picks highest score = A [OK]

Hint: Remember: dis_max picks the best single score, not sums [OK]

Common Mistakes:

• Thinking dis_max sums scores from all queries
• Confusing dis_max with filter queries
• Assuming dis_max sorts documents alphabetically

2. Which of the following is the correct syntax to use a dis_max query with two match queries in Elasticsearch?

easy

A. {\"dis_max\": {\"queries\": {\"match\": {\"field1\": \"value1\"}, \"match\": {\"field2\": \"value2\"}}}}

B. {\"dis_max\": [{\"match\": {\"field1\": \"value1\"}}, {\"match\": {\"field2\": \"value2\"}}]}

C. {\"dis_max\": {\"queries\": [{\"match\": {\"field1\": \"value1\"}}, {\"match\": {\"field2\": \"value2\"}}]}}

D. {\"dis_max\": {\"query\": [{\"match\": {\"field1\": \"value1\"}}, {\"match\": {\"field2\": \"value2\"}}]}}

Solution

1. Step 1: Check the dis_max query structure

   The dis_max query requires a queries array containing individual query objects.

2. Step 2: Validate the JSON syntax

   {\"dis_max\": {\"queries\": [{\"match\": {\"field1\": \"value1\"}}, {\"match\": {\"field2\": \"value2\"}}]}} correctly uses queries as an array with two match queries inside separate objects.

3. Final Answer:

   {"dis_max": {"queries": [{"match": {"field1": "value1"}}, {"match": {"field2": "value2"}}]}} -> Option C

4. Quick Check:

   dis_max uses queries array = D [OK]

Hint: dis_max needs a 'queries' array, not 'query' or object [OK]

Common Mistakes:

• Using 'query' instead of 'queries' key
• Passing queries as an object instead of an array
• Omitting the array brackets around queries

3. Given this Elasticsearch query snippet using dis_max with tie_breaker set to 0.3:

{
  "dis_max": {
    "queries": [
      { "match": { "title": "apple" } },
      { "match": { "description": "apple" } }
    ],
    "tie_breaker": 0.3
  }
}

What effect does the tie_breaker parameter have on the scoring?

medium

A. It adds 30% of the scores from non-highest matching queries to the final score

B. It multiplies the highest score by 0.3

C. It ignores all but the highest scoring query

D. It subtracts 0.3 from the highest score

Solution

1. Step 1: Understand tie_breaker in dis_max

   The tie_breaker adds a fraction of the scores from other matching queries to the highest score.

2. Step 2: Apply the 0.3 value meaning

   With tie_breaker 0.3, 30% of the other query scores are added to the highest score, boosting documents matching multiple queries.

3. Final Answer:

   It adds 30% of the scores from non-highest matching queries to the final score -> Option A

4. Quick Check:

   tie_breaker 0.3 adds 30% other scores = C [OK]

Hint: tie_breaker adds fraction of other scores to highest score [OK]

Common Mistakes:

• Thinking tie_breaker multiplies highest score directly
• Assuming tie_breaker ignores other queries
• Believing tie_breaker subtracts from scores

4. You wrote this dis_max query but it returns an error:

{
  "dis_max": {
    "queries": {
      { "match": { "field1": "value1" } },
      { "match": { "field2": "value2" } }
    }
  }
}

What is the error and how to fix it?

medium

A. The dis_max query requires a boost parameter

B. The queries value should be an array, not an object; replace curly braces with square brackets

C. The match queries must be inside a 'query' key, not 'queries'

D. The field names must be strings without quotes

Solution

1. Step 1: Identify the queries structure error

   The queries key must hold an array of query objects, but curly braces {} were used, making it an object.

2. Step 2: Correct the syntax

   Replace the curly braces around the queries with square brackets [] to form a valid array.

3. Final Answer:

   The queries value should be an array, not an object; replace curly braces with square brackets -> Option B

4. Quick Check:

   queries must be array [] not object {} = A [OK]

Hint: Use [] for queries array, not {} [OK]

Common Mistakes:

• Using curly braces instead of square brackets for queries
• Adding unnecessary boost parameter causing errors
• Removing quotes from field names

5. You want to boost the importance of a dis_max query compared to other queries in your Elasticsearch search. Which is the correct way to apply a boost to the entire dis_max query?

hard

A. { "dis_max": { "boost": 2.0, "tie_breaker": 0.5 } }

B. { "dis_max": { "queries": [...], "boost": "high" } }

C. { "dis_max": { "queries": [...], "tie_breaker": 2.0 } }

D. { "dis_max": { "queries": [...], "boost": 2.0 } }

Solution

1. Step 1: Understand boost usage in dis_max

   The boost parameter is applied at the dis_max query level to increase its overall importance.

2. Step 2: Validate correct boost syntax

   Boost must be a numeric value; { "dis_max": { "queries": [...], "boost": 2.0 } } correctly places boost: 2.0 inside dis_max alongside queries.

3. Final Answer:

   { "dis_max": { "queries": [...], "boost": 2.0 } } -> Option D

4. Quick Check:

   Boost is numeric and inside dis_max = B [OK]

Hint: Put numeric boost inside dis_max alongside queries [OK]

Common Mistakes:

• Using non-numeric boost values like strings
• Placing boost outside dis_max query
• Confusing tie_breaker with boost