Why Exists query in Elasticsearch? - Purpose & Use Cases

The Big Idea

What if you could instantly find every document that has the exact info you need, without searching one by one?

The Scenario

Imagine you have a huge list of documents, and you want to find only those that have a certain piece of information filled in, like an email address or a phone number.

The Problem

Checking each document one by one to see if that information exists is slow and tiring. It's easy to miss some or make mistakes, especially when the list is very large.

The Solution

The Exists query in Elasticsearch quickly finds all documents where a specific field is present. It saves time and effort by doing the hard work for you.

Before vs After

✗ Before

for doc in documents:
    if 'email' in doc:
        print(doc)

✓ After

{ "query": { "exists": { "field": "email" } } }

What It Enables

It lets you instantly find all records that contain the information you need, making your searches fast and accurate.

Real Life Example

For example, a company wants to send a newsletter only to users who have provided their email address. The Exists query helps find those users easily.

Key Takeaways

Manually checking for fields is slow and error-prone.

Exists query quickly finds documents with a specific field.

This makes searching large data sets simple and reliable.

Practice

1. What does the exists query do in Elasticsearch?

easy

A. Finds documents where a specific field is present

B. Finds documents where a field has a specific value

C. Deletes documents missing a field

D. Updates documents with a missing field

Solution

1. Step 1: Understand the purpose of the exists query

   The exists query checks if a field exists in documents, filtering those that have the field.

2. Step 2: Compare options with the query's function

   Only Finds documents where a specific field is present correctly describes this behavior; others describe different operations.

3. Final Answer:

   Finds documents where a specific field is present -> Option A

4. Quick Check:

   Exists query = finds documents with field [OK]

Hint: Exists query checks if a field is present in documents [OK]

Common Mistakes:

• Confusing exists query with term query
• Thinking it deletes or updates documents
• Assuming it checks field values

2. Which of the following is the correct syntax for an exists query in Elasticsearch?

easy

A. {"query": {"exists": {"field_name": "user"}}}

B. {"query": {"exists": "user"}}

C. {"query": {"exists": {"value": "user"}}}

D. {"query": {"exists": {"field": "user"}}}

Solution

1. Step 1: Recall the correct JSON structure for exists query

   The exists query requires a "field" key inside its object specifying the field name.

2. Step 2: Match options to correct syntax

   {"query": {"exists": {"field": "user"}}} matches the correct syntax: {"exists": {"field": "user"}}. Others use wrong keys or structure.

3. Final Answer:

   {"query": {"exists": {"field": "user"}}} -> Option D

4. Quick Check:

   Exists query syntax uses "field" key [OK]

Hint: Exists query needs "field" key inside its object [OK]

Common Mistakes:

• Using "value" or "field_name" instead of "field"
• Omitting the object braces around field
• Passing field as a string directly

3. Given the following Elasticsearch query, what documents will it return?

{"query": {"exists": {"field": "email"}}}

medium

A. Documents where the email field exists

B. Documents where the email field is missing

C. Documents where the email field is empty

D. All documents regardless of email field

Solution

1. Step 1: Analyze the query structure

   The query uses an exists query on the field "email", which filters documents having that field.

2. Step 2: Understand what exists means for documents

   Documents returned must have the "email" field present, regardless of its value.

3. Final Answer:

   Documents where the email field exists -> Option A

4. Quick Check:

   Exists query returns docs with field present [OK]

Hint: Exists query returns docs with the field present only [OK]

Common Mistakes:

• Thinking it returns docs missing the field
• Assuming empty fields are excluded
• Believing it returns all documents

4. Identify the error in this Elasticsearch exists query:

{"query": {"exists": {"field_name": "status"}}}

medium

A. The field value should be an array, not a string

B. The query should use "match" instead of "exists"

C. The key should be "field", not "field_name"

D. The query is missing a "filter" wrapper

Solution

1. Step 1: Check the key used inside exists query

   The exists query requires the key "field" to specify the field name.

2. Step 2: Compare the given key with the correct one

   The query uses "field_name" which is incorrect and causes an error.

3. Final Answer:

   The key should be "field", not "field_name" -> Option C

4. Quick Check:

   Exists query key = "field" [OK]

Hint: Use "field" key inside exists query, not "field_name" [OK]

Common Mistakes:

• Using incorrect key names inside exists query
• Confusing exists with match query
• Adding unnecessary wrappers

5. You want to find all documents where the field phone exists but is not empty or null. Which Elasticsearch query best achieves this?

hard

A. {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"term": {"phone": ""}}]}}}

B. {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"script": {"script": "doc['phone'].value != ''"}}]}}}

C. {"query": {"exists": {"field": "phone"}}}

D. {"query": {"exists": {"field": "phone"}, "term": {"phone": ""}}}

Solution

1. Step 1: Understand the requirement

   We want documents where "phone" exists and is not empty or null.

2. Step 2: Evaluate each option

   {"query": {"exists": {"field": "phone"}}} only checks existence, not emptiness. {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"term": {"phone": ""}}]}}} incorrectly requires phone to be empty. {"query": {"exists": {"field": "phone"}, "term": {"phone": ""}}} has invalid syntax combining exists and term at same level. {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"script": {"script": "doc['phone'].value != ''"}}]}}} uses a bool must with exists and a script checking phone is not empty, correctly filtering as needed.

3. Final Answer:

   {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"script": {"script": "doc['phone'].value != ''"}}]}}} -> Option B

4. Quick Check:

   Exists + script to exclude empty = {"query": {"bool": {"must": [{"exists": {"field": "phone"}}, {"script": {"script": "doc['phone'].value != ''"}}]}}} [OK]

Hint: Combine exists with script to exclude empty values [OK]

Common Mistakes:

• Using exists alone without checking empty values
• Incorrectly combining queries at same level
• Assuming term query with empty string works for exclusion