Scan vs Query performance comparison in DynamoDB - Visual Side-by-Side Comparison

Concept Flow - Scan vs Query performance comparison

Start Operation

↓

Use Partition Key

↓

Fetch Matching Items

↓

Return Results

↓

End Operation

Shows how Query uses keys to fetch matching items efficiently, while Scan reads the whole table and filters later.

Execution Sample

DynamoDB

Query: SELECT * FROM table WHERE partition_key = 'A'
Scan: SELECT * FROM table WHERE attribute = 'A'

Query fetches items by partition key directly; Scan reads all items and filters after.

Execution Table

Step	Operation	Items Read	Items Returned	Performance Impact
1	Query starts	0	0	Fast start, uses partition key
2	Query reads matching partition	3	3	Reads only relevant items
3	Query returns results	3	3	Efficient, low cost
4	Scan starts	0	0	Starts reading entire table
5	Scan reads all items	10	3	Reads all items, filters later
6	Scan returns filtered results	10	3	Slower, higher cost
7	End	-	-	Query is faster and cheaper than Scan

💡 Scan reads all 10 items, Query reads only 3 matching items; Query is more efficient.

Variable Tracker

Variable	Start	After Query Step 2	After Scan Step 5	Final
Items Read	0	3	10	10
Items Returned	0	3	3	3

Key Moments - 2 Insights

Why does Query read fewer items than Scan?

Because Query uses the partition key to directly access matching items (see execution_table row 2), while Scan reads the entire table (row 5) and filters afterward.

Does Scan always return more items than Query?

No, Scan can return the same number of items as Query (see rows 3 and 6), but it reads more items internally, making it slower and more costly.

Visual Quiz - 3 Questions

Look at the execution_table, how many items does Query read at step 2?

A 3

B 10

C 0

D 5

💡 Hint

Check the 'Items Read' column at step 2 in execution_table.

At which step does Scan finish reading all items?

A Step 4

B Step 5

C Step 6

D Step 3

💡 Hint

Look for 'Scan reads all items' in execution_table step 5.

If the table had 1000 items but only 3 matched the Query, how would Query's 'Items Read' change?

A It would read 1000 items

B It would read 0 items

C It would read 3 items

D It would read 500 items

💡 Hint

Query reads only matching partition key items, as shown in variable_tracker.

Concept Snapshot

Query uses partition key to fetch matching items directly.
Scan reads the entire table and filters afterward.
Query is faster and cheaper than Scan.
Use Query when you know the partition key.
Scan is costly and slower for large tables.
Always prefer Query for performance.

Full Transcript

This visual execution compares DynamoDB Query and Scan operations. Query uses the partition key to directly fetch matching items, reading fewer items and returning results quickly. Scan reads the entire table, then filters items, resulting in more items read and slower performance. The execution table shows Query reading 3 items and Scan reading 10 items to return the same 3 results. Variable tracking confirms Query reads fewer items. Key moments clarify why Query is more efficient and when Scan returns the same number of items but at higher cost. The quiz tests understanding of items read and performance differences. Remember, Query is best when you know the partition key; Scan should be avoided for large tables due to cost and speed.