The SET SESSION [NO]OLD_COPY_DN option controls the behavior when performing a formatted COPY FROM (read from file) with dummy column format specifiers.

A dummy column format is column_name=Dn where n is a number. With the old behavior, Dn caused a COPY FROM to read and discard the next n characters. For COPY INTO, however, Dn wrote n copies of the column name to the file. So, if the column name was more than one character a COPY INTO generated a file that a COPY FROM with the same format specifiers did not load correctly. The current default behavior is to read and discard (n * length of column name) on COPY FROM.