selection - how should the "select all" select when paginating elements

Suppose I have a list of 100+ pieces of data that I'm displaying in a table that's paginating 40 elements per page.

Each row has 'select' checkbox that enables the user to select/unselect an individual item. On top of the list, there's also a 'select/unselect all' checkbox. For the moment, I have the functionality designed so that when the user clicks that checkbox, the action of toggling the selection is performed only on the elements currently displayed in the page. What are the best practices on 'select all'?

Should the action operate on:

a) only the visible elements being paginated, or

b) the entire list?

Answer

Gmail does something that works pretty well, and has been around for a while.

Firstly, when you search for a particular string (in this case 'ebay' which gave me loads of results to work with) and then choose to select everything on the page (from the checkbox in the top-left) then it produces a message saying:

All 20 conversations on this page are selected. 'Select all conversations that match this search'

In the top-right is a '1-20 of many' text, but when there are a more reasonable number (say, 100 overall) then it'll tell you the actual number, and not just 'many'.

Then, if you choose that 'select all' message it updates your selection to show:

All conversations in this search are selected. Clear selection.

So give the user the ability to select only the ones on screen and also show how many extra ones are available. Give the user the ability to then toggle between the presently viewing list and the complete list before they decide they want to confirm the action.