Better Way to Deal with Data from Database and Lists

Question

1. I have TextField and onChange function it filters ListView, so is it better get whole item from table and save them in list then when change text filter that list or open stream get data from table every single change in search TextField?

2. I have lists in my app with elements, is it preferable when I go to another page or when I don't use the items, clear them every time or it is ok, it doesn't affect the app?