How can fetch and exec work inside parfor-loop

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TIANTIANBABA 2021-1-17

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https://ww2.mathworks.cn/matlabcentral/answers/719270-how-can-fetch-and-exec-work-inside-parfor-loop

编辑： TIANTIANBABA 2021-1-18

The following is my code: when change parfor to for, everything works fine, but when in parfor-loop, it won't work.

Can someone know how to let exec and fetch work inside a parfor-loop ？Thanks.

sqlStr = 'select echo,wave from vibdata';

parfor m = 1:size(vs_list_temp,1)

DatabaseExpo(xx1,'','',xx2,xx3,m,sqlStr); % let everything do in DatabaseExpo function

end

function DatabaseExpo(loca1,loca2,loca3,loca4,loca5,m,sqlStr)

eval(['conn_' num2str(m) ' = database(loca1,loca2,loca3,loca4,loca5);'])

eval(['curs_' num2str(m) ' = exec(conn_' num2str(m) ',sqlStr);'])

dataObj = fetch(eval(['curs_' num2str(m)]));

data = dataObj.Data;

close(eval(['curs_' num2str(m)]));

close(eval(['conn_' num2str(m)]));

save(['dbrjs' num2str(m)],'data');

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Edric Ellis 2021-1-18

在 MATLAB Online 中打开

What's the error you are getting? Are you sure your database supports multiple concurrent connections? Rather than using eval , you ought to be able to simply something more like this:

parfor m = ..
    out{m} = DatabaseExpo(..,m,..);
end
function data = DatabaseExpo(..)
    conn = database(..);
    curs = exec(conn, sqlStr);
    dataObj = fetch(curs);
    data = dataObj.Data;
    close(curs);
    close(conn);
end

TIANTIANBABA 2021-1-18

编辑：TIANTIANBABA 2021-1-18

Error message: fetch UndefinedFunction. ..................

Reason: Undefine struct type input of fetch function.

My database is SQLITE, and each of my file is a "standalone" database. Each of my file (database) is very small, just aboute 2M. But I got so many files.

About eval, I use the same expression as you did in function DatabaseExpo(..), but MATLAB gives out error message. I am not familiar with database operation, and I don't know whether use eval to give different conn names and curs names can help, so you see my code with eval expression. But unfortunatelly, it doesn't help.

I have try parallel.pool.Constant(conn), but it doesn't help, either.

Driver of the database is: sqlite-jdbc-3.21.0.jar

Thanks

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How can fetch and exec work inside parfor-loop

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