JSONB containment (@>) operator in PostgreSQL - Time & Space Complexity

Time Complexity: JSONB containment (@>) operator

O(n * m)

Understanding Time Complexity

We want to understand how the time it takes to check if one JSONB value contains another grows as the data size increases.

Specifically, how does the JSONB containment operator (@>) behave when the JSON data gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following PostgreSQL query using the JSONB containment operator.

SELECT *
FROM products
WHERE attributes @> '{"color": "red"}';
    

This query finds all products whose JSONB column attributes contains the key-value pair color: red.

Identify Repeating Operations

Look for repeated work inside the query execution.

• Primary operation: For each row, the database checks if the JSONB data contains the given JSON structure.
• How many times: This check happens once per row scanned in the table.

How Execution Grows With Input

As the number of rows grows, the database must do more containment checks.

Input Size (n rows)	Approx. Operations
10	10 containment checks
100	100 containment checks
1000	1000 containment checks

Pattern observation: The number of containment checks grows directly with the number of rows scanned.

Final Time Complexity

Time Complexity: O(n * m)

This means the time to run the query grows linearly with the number of rows checked and the size of the JSONB data per row.

Common Mistake

[X] Wrong: "The JSONB containment operator runs in constant time no matter how big the data is."

[OK] Correct: The operator must look inside each JSONB value, so the time depends on how many rows and the size of JSON data it checks.

Interview Connect

Understanding how JSONB containment scales helps you write efficient queries and explain performance in real projects.

Self-Check

What if we add a GIN index on the JSONB column? How would the time complexity change?

Practice

1.

What does the PostgreSQL @> operator do when used with JSONB data?

easy

A. Checks if the right JSONB contains the left JSONB

B. Deletes the right JSONB from the left JSONB

C. Merges two JSONB objects into one

D. Checks if the left JSONB contains the right JSONB

Solution

1. Step 1: Understand the operator's purpose

   The @> operator in PostgreSQL is used to check containment between JSONB values.

2. Step 2: Identify the direction of containment

   It checks if the JSONB on the left side contains the JSONB on the right side.

3. Final Answer:

   Checks if the left JSONB contains the right JSONB -> Option D

4. Quick Check:

   JSONB containment = left contains right [OK]

Hint: Remember: left @> right means left contains right [OK]

Common Mistakes:

• Confusing which side contains which
• Thinking it merges JSONB objects
• Assuming it deletes data

2.

Which of the following is the correct syntax to check if a JSONB column data contains the JSON {"key": "value"}?

SELECT * FROM table WHERE data ___ '{"key": "value"}'::jsonb;

easy

A. @>

B. <@

C. ->

D. ->>

Solution

1. Step 1: Identify the containment operator

   The @> operator checks if the left JSONB contains the right JSONB.

2. Step 2: Apply correct syntax

   To check if data contains the JSON, use data @> '{"key": "value"}'::jsonb.

3. Final Answer:

   @> -> Option A

4. Quick Check:

   Containment operator = @> [OK]

Hint: Use @> with right side cast to jsonb [OK]

Common Mistakes:

• Using <@ which reverses containment
• Using -> or ->> which access keys, not contain
• Forgetting to cast string to jsonb

3.

Given the table items with a JSONB column attributes, what will this query return?

SELECT id FROM items WHERE attributes @> '{"color": "red"}'::jsonb;

Assuming items contains:

id | attributes
---+-------------------------
1  | {"color": "red", "size": "M"}
2  | {"color": "blue", "size": "L"}
3  | {"color": "red"}
4  | {"size": "S"}

medium

A. [4]

B. [2, 4]

C. [1, 3]

D. [1, 2, 3]

Solution

1. Step 1: Understand the query condition

   The query selects rows where attributes contain the JSON {"color": "red"}.

2. Step 2: Check each row's attributes

   Rows 1 and 3 have "color": "red"; rows 2 and 4 do not.

3. Final Answer:

   [1, 3] -> Option C

4. Quick Check:

   Only rows with color red = [1, 3] [OK]

Hint: Look for rows where JSON contains the key-value pair exactly [OK]

Common Mistakes:

• Including rows without the key-value pair
• Confusing @> with key existence operator
• Ignoring the JSONB cast requirement

4.

Identify the error in this query that tries to find rows where info JSONB column contains {"active": true}:

SELECT * FROM users WHERE info @> '{active: true}';

medium

A. Wrong operator used, should be <@

B. JSON keys must be in double quotes

C. Missing cast to jsonb for the right side

D. Boolean true must be in quotes

Solution

1. Step 1: Check JSON syntax correctness

   JSON keys must be enclosed in double quotes, so active should be "active".

2. Step 2: Verify casting and operator

   The query is missing ::jsonb cast, but the main syntax error is the missing quotes around the key.

3. Final Answer:

   JSON keys must be in double quotes -> Option B

4. Quick Check:

   JSON keys need double quotes [OK]

Hint: Always quote JSON keys and cast string to jsonb [OK]

Common Mistakes:

• Forgetting double quotes around keys
• Not casting string to jsonb
• Using wrong containment operator

5.

You want to find all rows in products where the details JSONB column contains {"features": {"waterproof": true}} and also the price is less than 100. Which query is correct?

hard

A. SELECT * FROM products WHERE details @> '{"features": {"waterproof": true}}'::jsonb AND price < 100;

B. SELECT * FROM products WHERE details <@ '{"features": {"waterproof": true}}'::jsonb AND price < 100;

C. SELECT * FROM products WHERE details @> '{features: {waterproof: true}}' AND price < 100;

D. SELECT * FROM products WHERE details -> 'features' @> '{"waterproof": true}' AND price < 100;

Solution

1. Step 1: Use correct containment operator and JSON syntax

   The @> operator checks if details contains the given JSON. The JSON must have keys in double quotes and be cast to jsonb.

2. Step 2: Combine with price condition

   Use AND price < 100 to filter by price.

3. Final Answer:

   SELECT * FROM products WHERE details @> '{"features": {"waterproof": true}}'::jsonb AND price < 100; -> Option A

4. Quick Check:

   Correct operator, JSON syntax, and price filter [OK]

Hint: Use @> with correct JSON and add AND for price filter [OK]

Common Mistakes:

• Using <@ instead of @>
• Missing quotes in JSON keys
• Incorrect JSONB casting
• Trying to use -> operator for containment