How to retrieve additional data for grid row to ed

Question

How to retrieve additional data for grid row to edit only if user want to edit certain row?

In other words:

• grid is defined with datasource with f.e. 4 fields,
• but with pressing edit row I want popup to retrieve 50 more fields for this row,
• BUT after pressing update I want grid to show updated data (with this 4 fields)

In other words:

• basic data for datasource for grid contain 4 fields/colums
• after clicking edit for row this datasource adds more fields to datasource for edit row.

I know I can at start retrieve all 54 fields and show only 4 of them in the grid and all of them in edit popup, but this might cause perfomance problem.