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    %% Evaluate Deferred Tall Array Calculation
% Create a datastore for the |airlinesmall.csv| data set. Select a subset
% of variables to work with, and treat |'NA'| values as missing data so
% that |datastore| replaces them with |NaN| values. Convert the datastore
% into a tall table.
varnames = {'Year','ArrDelay','UniqueCarrier'};
ds = datastore('airlinesmall.csv', 'TreatAsMissing', 'NA',...
    'SelectedVariableNames',varnames);
T = tall(ds)

%%
% Calculate the size of the tall table.
sz = size(T)

%%
% MATLAB(R) does not immediately evaluate most operations on tall arrays.
% Instead, MATLAB remembers the operations you perform as you enter them
% and optimizes the calculations in the background. 
%
% When you use |gather| on an unevaluated tall array, MATLAB executes all
% of the queued operations using the minimum number of passes through the
% data. This optimization greatly reduces the execution time of large
% calculations. For this reason, you should use |gather| only when you need
% to see a result. 

%%
% Use |gather| to execute the calculation and collect the result into
% memory.
S = gather(sz)