Indexed parameters in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Indexed parameters

Define event with indexed params

↓

Emit event with values

↓

Blockchain stores event

↓

Indexed params stored as topics

↓

Listeners filter events by topics

↓

Retrieve matching events efficiently

Indexed parameters in blockchain events allow quick filtering by storing key values as topics, making event search efficient.

Execution Sample

Blockchain / Solidity

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

function send(address to, uint256 amount) public {
    emit Transfer(msg.sender, to, amount);
}

This code defines a Transfer event with two indexed parameters and emits it when sending tokens.

Execution Table

Step	Action	Indexed Params (topics)	Non-indexed Params (data)	Result
1	Define event Transfer with indexed 'from' and 'to'	from, to	value	Event signature and topics set
2	Call send() with to=0xABC, amount=100			Function executes
3	Emit Transfer event	msg.sender, to	amount	Event logged with topics and data
4	Blockchain stores event	Topics: from, to addresses	Data: value=100	Event stored in logs
5	Listener filters by 'from' address	Matches topic 'from'		Event retrieved efficiently
6	Listener filters by 'value'		Cannot filter efficiently	Full scan needed
7	End			Event emission and indexing complete

💡 Event emitted and indexed parameters stored as topics for efficient filtering

Variable Tracker

Variable	Start	After emit	Final
from (indexed)	undefined	msg.sender	msg.sender
to (indexed)	undefined	to	to
value (non-indexed)	undefined	amount	amount

Key Moments - 3 Insights

Why are some parameters marked as indexed in events?

Indexed parameters are stored as topics, allowing fast filtering by listeners, as shown in execution_table step 5.

Can non-indexed parameters be used for efficient event filtering?

No, non-indexed parameters are stored in data and require scanning all events, as shown in execution_table step 6.

How many parameters can be indexed in an event?

Up to three parameters can be indexed per event, enabling multiple filters as in the example with two indexed parameters.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what are the indexed parameters stored as?

A Topics

B Data

C Function arguments

D Transaction inputs

💡 Hint

Check the 'Indexed Params (topics)' column at step 3 in execution_table

At which step does the listener efficiently filter events by an indexed parameter?

A Step 2

B Step 6

C Step 5

D Step 7

💡 Hint

Look at the 'Result' column describing filtering in execution_table

If the 'value' parameter was indexed, how would filtering change at step 6?

A Filtering would still require full scan

B Filtering would be efficient using topics

C Filtering would be impossible

D Filtering would be slower

💡 Hint

Compare step 6 filtering with indexed vs non-indexed parameters in execution_table

Concept Snapshot

Indexed parameters in blockchain events:
- Marked with 'indexed' keyword
- Stored as topics (up to 3)
- Enable fast event filtering
- Non-indexed stored in data, slower to filter
- Used in event definitions and emissions

Full Transcript

Indexed parameters in blockchain events are special parameters marked with the 'indexed' keyword. When an event is emitted, these indexed parameters are stored as topics, which are like tags that help listeners quickly find events matching certain criteria. For example, in a Transfer event with 'from' and 'to' addresses indexed, listeners can filter events by these addresses efficiently. Non-indexed parameters like 'value' are stored in the event data and require scanning all events to filter. Up to three parameters can be indexed per event. This makes event searching faster and more efficient on the blockchain.