When the Data Source definition is updated, the bottom main area shows a data preview :
Loading more data rows
In order to improve performance while editing the Data Source, only a few data rows are fetched.
By default, only the first 100 data rows are fetched.
If your data set contains more than 100 rows, you can click the “Load More” button on the top right to get up to 1000 rows.
Then if your data set exceeds 1000 rows, the Data Preview will still limit to 1000 rows :
Refreshing the Data Preview
You can force to get new data by clicking the Refresh button on the top toolbar :