Speed up file write taking weeks

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PK_A_Field    Data_A_Field
       D                     x
       D                     y
       E                      t

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PK_B_Field    Data_B_Field
       D                     m
...
       D                     o
       E                     m
       E                     s

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PK_Field     Data_A_Field    Data_B_Field
     D                      x                     m
...
     D                      y                     o
     E                       t                     m
     E                       t                     s

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