Filtering by field values in Rest API - Time & Space Complexity

Time Complexity: Filtering by field values

O(n)

Understanding Time Complexity

When filtering data by field values in a REST API, we want to know how the time to get results changes as the data grows.

We ask: How does the filtering process scale with more data?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

GET /items?color=red

// Server-side pseudo-code
items = database.getAllItems()
filtered = []
for item in items:
    if item.color == 'red':
        filtered.append(item)
return filtered
    

This code fetches all items and filters those whose color field equals 'red'.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through all items to check their color.
• How many times: Once for each item in the data set.

How Execution Grows With Input

As the number of items grows, the server checks each one once.

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

Pattern observation: The work grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to filter grows in a straight line as the data size grows.

Common Mistake

[X] Wrong: "Filtering by a field is instant no matter how many items there are."

[OK] Correct: The server must check each item to see if it matches, so more items mean more work.

Interview Connect

Understanding how filtering scales helps you explain API performance and design better data queries.

Self-Check

"What if the data was stored in a database with an index on the color field? How would the time complexity change?"

Practice

1. What does filtering by field values in a REST API usually do?

easy

A. Returns all data entries without any filtering

B. Deletes data entries that match the specified field values

C. Updates all data entries regardless of field values

D. Returns only the data entries that match the specified field values

Solution

1. Step 1: Understand filtering purpose

   Filtering is used to get only data that matches certain criteria, not to delete or update.

2. Step 2: Identify filtering effect

   Filtering by field values returns only entries matching those values.

3. Final Answer:

   Returns only the data entries that match the specified field values -> Option D

4. Quick Check:

   Filtering = Select matching data [OK]

Hint: Filtering narrows data to matching entries only [OK]

Common Mistakes:

• Thinking filtering deletes data
• Confusing filtering with updating data
• Assuming filtering returns all data

2. Which of the following is the correct way to filter users by age 30 in a REST API URL?

easy

A. /users?age=30

B. /users/age=30

C. /users?filter=age>30

D. /users/age/30

Solution

1. Step 1: Recognize query parameter syntax

   Filtering uses query parameters after ? with key=value pairs.

2. Step 2: Match correct syntax

   /users?age=30 correctly filters users with age 30.

3. Final Answer:

   /users?age=30 -> Option A

4. Quick Check:

   Query param = ?key=value [OK]

Hint: Use ?field=value to filter in REST API URLs [OK]

Common Mistakes:

• Using slashes instead of query parameters
• Using incorrect filter syntax like > without API support
• Missing the ? before query parameters

3. Given this API call: /products?category=books&available=true, what data will the API return?

medium

A. All products regardless of category or availability

B. Only products in the 'books' category that are available

C. Only products that are unavailable

D. Only products in the 'books' category regardless of availability

Solution

1. Step 1: Analyze query parameters

   The URL filters products where category equals 'books' and available equals true.

2. Step 2: Determine filtering effect

   Both conditions must be met, so only available books are returned.

3. Final Answer:

   Only products in the 'books' category that are available -> Option B

4. Quick Check:

   Multiple filters = AND condition [OK]

Hint: Multiple filters combine with AND by default [OK]

Common Mistakes:

• Ignoring one of the filter conditions
• Assuming filters combine with OR
• Confusing availability true/false

4. You want to filter orders by status 'shipped' using /orders?status=shipped, but the API returns all orders. What is the likely problem?

medium

A. The query parameter should be ?state=shipped instead

B. The URL is missing a trailing slash

C. The API does not support filtering by 'status'

D. The filter value 'shipped' is case-sensitive and should be uppercase

Solution

1. Step 1: Check API documentation for supported filters

   If 'status' is not a supported filter field, the API ignores it and returns all data.

2. Step 2: Evaluate other options

   Changing parameter name or case only works if API supports it; trailing slash usually doesn't affect filtering.

3. Final Answer:

   The API does not support filtering by 'status' -> Option C

4. Quick Check:

   Unsupported filter = no effect [OK]

Hint: Check API docs for allowed filter fields first [OK]

Common Mistakes:

• Assuming any field can be filtered
• Changing URL format without docs
• Ignoring case sensitivity without confirmation

5. You have an API endpoint /employees that supports filtering by department and active status. How would you get all active employees in the 'sales' department?

hard

A. /employees?department=sales&active=true

B. /employees?active=true,department=sales

C. /employees?department=sales|active=true

D. /employees?department=sales;active=true

Solution

1. Step 1: Use correct query parameter syntax

   Query parameters are joined by & and use key=value pairs.

2. Step 2: Combine filters for department and active status

   /employees?department=sales&active=true correctly filters active sales employees.

3. Final Answer:

   /employees?department=sales&active=true -> Option A

4. Quick Check:

   Use & to combine filters [OK]

Hint: Combine filters with & and key=value pairs [OK]

Common Mistakes:

• Using | or ; instead of & to join filters
• Mixing order of parameters incorrectly
• Using invalid separators causing syntax errors