Bool query in depth in Elasticsearch - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a bool query in Elasticsearch?

A bool query lets you combine multiple queries using logical operators like must, should, must_not, and filter. It helps build complex search conditions.

beginner

Explain the must clause in a bool query.

The must clause means all queries inside it must match the document for it to be included in results. Think of it as an AND condition.

beginner

What does the should clause do in a bool query?

The should clause means at least one of the queries inside should match. It acts like an OR condition and can boost relevance if matched.

intermediate

How does the filter clause differ from must in a bool query?

The filter clause also requires matching documents but does not affect scoring. It is faster because it caches results and is used for filtering without relevance impact.

beginner

What is the role of must_not in a bool query?

must_not excludes documents that match the queries inside it. It works like a NOT condition to remove unwanted results.

Which bool query clause excludes documents from the results?

A should

B must

C must_not

D filter

The must_not clause excludes documents that match its queries.

Which clause in a bool query does NOT affect the relevance score?

A must

B filter

C must_not

D should

The filter clause filters documents without changing their relevance score.

If you want all conditions to be true for a document, which clause do you use?

A must

B should

C filter

D must_not

The must clause requires all its queries to match, like an AND condition.

What happens if a bool query has multiple should clauses but no must or filter clauses?

A At least one should clause must match

B No documents will match

C All should clauses must match

D Should clauses are ignored

When only should clauses exist, at least one should clause must match.

Which clause is best for caching and improving performance in bool queries?

A must_not

B should

C must

D filter

Filter clauses are cached and improve query performance.

Describe the four main clauses of a bool query and their roles.

Think about AND, OR, NOT, and filtering without scoring.

Explain how the filter clause improves query performance compared to must.

Focus on caching and scoring differences.

Practice

1. Which clause in an Elasticsearch bool query is used to exclude documents from the search results?

easy

A. must

B. should

C. must_not

D. filter

Solution

1. Step 1: Understand bool query clauses

   The bool query has four main clauses: must (required matches), filter (fast filtering), should (optional matches), and must_not (exclusions).

2. Step 2: Identify exclusion clause

   The must_not clause excludes documents that match its conditions from the results.

3. Final Answer:

   must_not -> Option C

4. Quick Check:

   Exclusion = must_not [OK]

Hint: Exclude with must_not clause in bool query [OK]

Common Mistakes:

• Confusing must_not with should
• Using filter to exclude documents
• Thinking must excludes documents

2. Which of the following is the correct syntax to add a filter clause inside a bool query in Elasticsearch?

easy

A. "bool": { "must": [ { "term": { "status": "active" } } ] }

B. "bool": { "must_not": { "term": { "status": "active" } } }

C. "bool": { "should": { "term": { "status": "active" } } }

D. "bool": { "filter": [ { "term": { "status": "active" } } ] }

Solution

1. Step 1: Recall bool query syntax for filter

   The filter clause expects an array of queries inside the bool query, e.g., "filter": [ { ... } ].

2. Step 2: Match correct syntax

   "bool": { "filter": [ { "term": { "status": "active" } } ] } correctly uses "filter": [ { "term": { "status": "active" } } ] which is valid syntax.

3. Final Answer:

   "bool": { "filter": [ { "term": { "status": "active" } } ] } -> Option D

4. Quick Check:

   Filter clause uses array syntax [OK]

Hint: Filter clause always uses an array of queries [OK]

Common Mistakes:

• Using object instead of array for filter
• Confusing must_not or should syntax
• Missing brackets around filter queries

3. Given this bool query snippet, what documents will it match?

{ "bool": { "must": [ { "match": { "title": "python" } } ], "should": [ { "term": { "tags": "programming" } } ], "must_not": [ { "term": { "status": "draft" } } ] } }

medium

A. Documents with title containing 'python', optionally tagged 'programming', excluding drafts

B. Documents tagged 'programming' only, ignoring title and status

C. Documents with status 'draft' and title 'python'

D. Documents with title 'python' or status 'draft'

Solution

1. Step 1: Analyze must clause

   The must clause requires documents to have 'python' in the title.

2. Step 2: Analyze should and must_not clauses

   The should clause boosts documents tagged 'programming' but is optional. The must_not clause excludes documents with status 'draft'.

3. Final Answer:

   Documents with title containing 'python', optionally tagged 'programming', excluding drafts -> Option A

4. Quick Check:

   Must + should + must_not = Documents with title containing 'python', optionally tagged 'programming', excluding drafts [OK]

Hint: Must is required, should is optional, must_not excludes [OK]

Common Mistakes:

• Thinking should is required
• Including documents with status 'draft'
• Ignoring must clause

4. Identify the error in this bool query snippet:

{ "bool": { "must": { "term": { "user": "kimchy" } }, "filter": { "term": { "status": "active" } } } }

medium

A. Missing "should" clause

B. "must" and "filter" should be arrays, not single objects

C. Using "term" inside bool is invalid

D. The query is correct as is

Solution

1. Step 1: Check bool clause types

   In bool queries, must and filter expect arrays of queries, not single objects.

2. Step 2: Identify syntax error

   Here, both must and filter are single objects, which is invalid syntax.

3. Final Answer:

   "must" and "filter" should be arrays, not single objects -> Option B

4. Quick Check:

   Bool clauses require arrays [OK]

Hint: Bool clauses must and filter need arrays [OK]

Common Mistakes:

• Using single object instead of array
• Forgetting to wrap queries in []
• Assuming should is mandatory

5. You want to find documents where the category is 'books' and either the author is 'Alice' or the year is after 2015, but exclude any documents marked as archived. Which bool query structure correctly implements this?

hard

A. { "bool": { "must": [ { "term": { "category": "books" } } ], "should": [ { "term": { "author": "Alice" } }, { "range": { "year": { "gt": 2015 } } } ], "must_not": [ { "term": { "status": "archived" } } ] } }

B. { "bool": { "filter": [ { "term": { "category": "books" } }, { "term": { "author": "Alice" } } ], "must_not": [ { "term": { "status": "archived" } } ], "should": [ { "range": { "year": { "gt": 2015 } } } ] } }

C. { "bool": { "must": [ { "term": { "category": "books" } }, { "term": { "author": "Alice" } } ], "must_not": [ { "term": { "status": "archived" } } ], "should": [ { "range": { "year": { "gt": 2015 } } } ] } }

D. { "bool": { "must": [ { "term": { "category": "books" } } ], "filter": [ { "term": { "author": "Alice" } }, { "range": { "year": { "gt": 2015 } } } ], "must_not": [ { "term": { "status": "archived" } } ] } }

Solution

1. Step 1: Identify required and optional conditions

   The category must be 'books' (required), and either author is 'Alice' or year > 2015 (optional, so use should).

2. Step 2: Exclude archived documents

   Use must_not to exclude documents with status 'archived'.

3. Step 3: Match query structure

   { "bool": { "must": [ { "term": { "category": "books" } } ], "should": [ { "term": { "author": "Alice" } }, { "range": { "year": { "gt": 2015 } } } ], "must_not": [ { "term": { "status": "archived" } } ] } } correctly uses must for category, should for author or year, and must_not for exclusion.

4. Final Answer:

   Option A's bool query structure -> Option A

5. Quick Check:

   Must + should + must_not for conditions [OK]

Hint: Use must for required, should for alternatives, must_not to exclude [OK]

Common Mistakes:

• Putting author and year in must (both required)
• Using filter instead of should for alternatives
• Missing must_not for exclusion