Indexed parameters in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Indexed parameters

O(n)

Understanding Time Complexity

When working with blockchain events, indexed parameters help filter data efficiently.

We want to understand how the cost of searching events grows as more data is added.

Scenario Under Consideration

Analyze the time complexity of the following event filter code using indexed parameters.

event Transfer(address indexed from, address indexed to, uint256 amount);

function getTransfersBySender(address sender) public view returns (Transfer[] memory) {
  // Filter events where 'from' equals sender
  return filterEvents("Transfer", "from", sender);
}
    

This code filters blockchain Transfer events by the sender address using an indexed parameter.

Identify Repeating Operations

Look for repeated checks or searches in the event logs.

• Primary operation: Searching event logs for matching indexed parameter values.
• How many times: Once per event stored in the blockchain logs.

How Execution Grows With Input

As the number of events grows, the search cost changes depending on indexing.

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

Pattern observation: Without indexing, operations grow linearly with events; with indexed parameters, filtering is faster because the blockchain organizes data for quick lookup.

Final Time Complexity

Time Complexity: O(n)

This means the time to find matching events grows in direct proportion to the number of events stored.

Common Mistake

[X] Wrong: "Indexed parameters make event searches instant regardless of data size."

[OK] Correct: Indexed parameters speed up filtering but still require checking entries; the cost grows with more events.

Interview Connect

Understanding how indexed parameters affect search cost shows you know how blockchain organizes data efficiently, a useful skill for real projects.

Self-Check

"What if we added multiple indexed parameters to the event? How would that affect the time complexity of filtering?"

Practice

1.

What is the main purpose of indexed parameters in blockchain events?

easy

A. To limit the size of event data

B. To encrypt event data for security

C. To tag event data for fast searching on the blockchain

D. To automatically execute functions on event trigger

Solution

1. Step 1: Understand the role of indexed parameters

   Indexed parameters are used to tag event data so it can be searched quickly on the blockchain.

2. Step 2: Compare with other options

   Encryption, size limits, or automatic execution are not related to indexed parameters.

3. Final Answer:

   To tag event data for fast searching on the blockchain -> Option C

4. Quick Check:

   Indexed parameters = fast search tags [OK]

Hint: Indexed means fast searchable tags in events [OK]

Common Mistakes:

• Thinking indexed encrypts data
• Confusing indexing with size limits
• Assuming indexed triggers functions

2.

Which of the following is the correct syntax to declare an indexed parameter in a Solidity event?

event Transfer(address ? from, address to, uint amount);

easy

A. indexed

B. index

C. indexing

D. indexedParam

Solution

1. Step 1: Recall Solidity event syntax

   The keyword to mark a parameter as indexed is exactly indexed.

2. Step 2: Check other options

   Other words like index, indexing, or indexedParam are invalid in Solidity event declarations.

3. Final Answer:

   indexed -> Option A

4. Quick Check:

   Use 'indexed' keyword for parameters [OK]

Hint: Use exact keyword 'indexed' for event parameters [OK]

Common Mistakes:

• Using 'index' instead of 'indexed'
• Adding extra words like 'indexedParam'
• Omitting the keyword entirely

3.

Given the Solidity event declaration:

event Approval(address indexed owner, address indexed spender, uint256 value);

Which parameters are indexed and can be filtered in event logs?

medium

A. owner and spender

B. owner and value

C. spender and value

D. owner, spender, and value

Solution

1. Step 1: Identify indexed parameters in the event

   The event declares owner and spender as indexed parameters.

2. Step 2: Understand filtering capability

   Only indexed parameters can be filtered in event logs, so owner and spender are filterable.

3. Final Answer:

   owner and spender -> Option A

4. Quick Check:

   Indexed parameters = owner, spender [OK]

Hint: Only parameters with 'indexed' keyword are filterable [OK]

Common Mistakes:

• Assuming all parameters are indexed
• Including non-indexed 'value' as filterable
• Confusing parameter order with indexing

4.

What is wrong with this Solidity event declaration?

event Deposit(address indexed from, uint indexed amount, uint indexed message, uint indexed timestamp);

medium

A. Indexed keyword is missing

B. Parameters must be public

C. Event name is invalid

D. More than 3 parameters are indexed

Solution

1. Step 1: Count indexed parameters

   The event has four indexed parameters: from, amount, message, and timestamp.

2. Step 2: Recall indexing limit

   Solidity allows a maximum of 3 indexed parameters per event. This declaration exceeds that limit.

3. Final Answer:

   More than 3 parameters are indexed -> Option D

4. Quick Check:

   4 indexed > max 3 [OK]

Hint: Max 3 indexed params per event [OK]

Common Mistakes:

• Thinking 4 indexed parameters are allowed
• Ignoring the 3 indexed parameter limit
• Confusing event name rules

5.

You want to create an event to track token transfers with these details: sender address, receiver address, amount, and a note message. You want to filter events by sender and receiver. Which event declaration is correct?

hard

A. event Transfer(address indexed sender, address indexed receiver, uint indexed amount, string indexed note);

B. event Transfer(address indexed sender, address indexed receiver, uint amount, string note);

C. event Transfer(address indexed sender, address receiver, uint indexed amount, string note);

D. event Transfer(address sender, address receiver, uint indexed amount, string indexed note);

Solution

1. Step 1: Identify which parameters need indexing

   Sender and receiver addresses must be indexed for filtering.

2. Step 2: Check indexing limits and types

   Only 3 parameters can be indexed. Amount can be indexed but note (string) cannot be indexed.

3. Step 3: Evaluate each option

   event Transfer(address indexed sender, address indexed receiver, uint amount, string note); indexes sender and receiver only, which is correct. event Transfer(address sender, address receiver, uint indexed amount, string indexed note); indexes amount and note incorrectly. event Transfer(address indexed sender, address receiver, uint indexed amount, string note); indexes amount but not receiver. event Transfer(address indexed sender, address indexed receiver, uint indexed amount, string indexed note); indexes 4 parameters which is invalid and string cannot be indexed.

4. Final Answer:

   event Transfer(address indexed sender, address indexed receiver, uint amount, string note); -> Option B

5. Quick Check:

   Index sender and receiver only; string can't be indexed [OK]

Hint: Index only filterable params; strings cannot be indexed [OK]

Common Mistakes:

• Indexing string parameters
• Indexing more than 3 parameters
• Not indexing all filterable parameters