The client requests a specific page; the server calculates which items to fetch, queries the database, and returns only that page's data with info for navigation.
Execution Sample
FastAPI
from fastapi import FastAPI, Query
app = FastAPI()
@app.get('/items/')
asyncdef read_items(page: int = 1, size: int = 10):
offset = (page - 1) * size
return {'offset': offset, 'limit': size}
This FastAPI endpoint calculates offset and limit for pagination based on page and size query parameters.
Execution Table
Step
Input (page, size)
Calculate offset
Calculate limit
Return value
1
page=1, size=10
(1-1)*10 = 0
10
{'offset': 0, 'limit': 10}
2
page=2, size=10
(2-1)*10 = 10
10
{'offset': 10, 'limit': 10}
3
page=3, size=5
(3-1)*5 = 10
5
{'offset': 10, 'limit': 5}
4
page=0, size=10
(0-1)*10 = -10
10
{'offset': -10, 'limit': 10}
5
page=1, size=0
(1-1)*0 = 0
0
{'offset': 0, 'limit': 0}
6
page=1, size=10
(1-1)*10 = 0
10
{'offset': 0, 'limit': 10}
💡 Execution ends after returning offset and limit for given page and size parameters.
Variable Tracker
Variable
Start
After 1
After 2
After 3
After 4
After 5
Final
page
1
1
2
3
0
1
1
size
10
10
10
5
10
0
10
offset
0
0
10
10
-10
0
0
limit
10
10
10
5
10
0
10
Key Moments - 3 Insights
Why does offset become negative when page is 0?
Because offset is calculated as (page - 1) * size, if page is 0, offset is (0-1)*size = -size, which is negative and usually invalid for database queries. See execution_table row 4.
What happens if size is 0?
Limit becomes 0, meaning no items will be fetched. This might cause empty results. See execution_table row 5.
Why do we calculate offset as (page - 1) * size?
Because page 1 should start at item 0, so offset 0 means start from first item. Page 2 starts after first page items, so offset is size, etc. See execution_table rows 1-3.
Visual Quiz - 3 Questions
Test your understanding
Look at the execution_table, what is the offset when page=3 and size=5?
A10
B5
C15
D0
💡 Hint
Check execution_table row 3 under 'Calculate offset'
At which step does the offset become negative?
AStep 2
BStep 4
CStep 3
DStep 5
💡 Hint
Look for negative offset value in execution_table
If size changes to 20 at page 1, what will the offset be?
Pagination in FastAPI:
- Use query params: page (default 1), size (default 10)
- Calculate offset = (page - 1) * size
- Use offset and limit in DB query
- Return subset of data
- Handle edge cases: page < 1 or size <= 0
- Provide metadata for client navigation
Full Transcript
This visual execution shows how pagination works in FastAPI. The client sends page and size parameters. The server calculates offset as (page - 1) times size to know where to start fetching items. Limit is the number of items per page. The server returns these values to fetch a subset of data. The execution table traces different inputs and their offset and limit results. Negative offsets happen if page is less than 1, which is usually invalid. Size zero means no items fetched. This pattern helps clients request data page by page efficiently.