save with "'-append" on table increase file size without modification of the variable

I create a table of 100x100, then save it:

t = array2table(rand(100));
save('table.mat', 't')

The file weight 76 Ko. Then, without changing t, I re-save it with append:

save('table.mat', 't', '-append')

The file now weight 152Ko (76*2), and it will add 76Ko each time I append the same table.

If you load the table, it's the same than the original one:

s = load('table.mat', 't');
setdiff(s.t, t) % It's empty

The documentation do say "save(filename,variables,"-append") adds new variables to an existing file. If a variable already exists in a MAT-file, then save overwrites it with the value in the workspace.", but the size of the MAT-File increase by the size of the table each time.

What is happening with the -append option ?

[dpb]

I can confirm a similar behavior w/ R2021b; it appears as though the '-append' option for a table (perhaps any compound class?) adds some overhead to the .mat file. Whether this is a bug or a feature, I think only a bug/support request to Mathworks will be able to answer.

[Stephen23]

The -append option does not remove or rearrange any existing data from the mat file. The reason is simply that rearranging/overwriting existing data would take time: potentially much more time than it takes to write your new data.

It is not a bug and is the same for all MATLAB versions.

A search of this forum will bring up several existing threads that discuss this in detail.

[Walter Roberson]

save -append works by first appending the new data to the .mat file, and then going back through the .mat file and marking previous versions of the variable as being unused.

[Arthur Roué]

Thanks all for your replies, I missed out the existing threads.

@Madheswaran provides a link to the bug report, this issue can be close.

Answer 1

[Madheswaran]

Hello,

The behaviour you are encountering is a known issue. When saving certain MATLAB data types—like tabular classes, strings, and non-fundamental types—using the ‘-append’ flag, the MAT-file size can increase with each save operation. This is a documented bug, and you can find more details in the MathWorks bug report here:

https://mathworks.com/support/bugreports/details/3166096

The report also provides two ways to work around this issue. Please refer to the link for more information.

Hope this helps!