Projecting into KeyValuePair via EF / Linq

Select only columnA and columnB from your table, and move further processing in memory:

return context.myTable
              .Select(o => new { o.columnA, o.columnB }) // only two fields
              .AsEnumerable() // to clients memory
              .Select(o => new KeyValuePair<int, string>(o.columnA, o.columnB))
              .ToList();

Consider also to create dictionary which contains KeyValuePairs:

return context.myTable.ToDictionary(o => o.columnA, o => o.columnB).ToList();