copy command in postgresql to append data
Assuming a Unix system

with PostgreQSL 9.3 or newer

Use the PROGRAM clause with cat as the program.

execute 'copy (select * from tableName) to PROGRAM ''cat &gt;&gt;/tmp/result.txt'''


This would append the results to the end of /tmp/result.txt instead of overwriting the file.

with older versions of PostgreSQL

A named pipe can be used by a separate process to concatenate results. 
Example:

$ mkfifo /tmp/pgfifo
$ while true; do cat /tmp/pgfifo &gt;&gt;/tmp/result.txt; done


This last command will block. Let it run until all the results are accumulated, then it can be terminated with ^C or kill.

In SQL, the fifo has to be fed with:

execute 'copy (select * from tableName) to ''/tmp/pgfifo'''