В MySQL / MariaDB:

SELECT timestamp, created_at, peer, txt FROM archive WHERE username = 'USER' and txt != '' INTO OUTFILE '/tmp/USER.csv' FIELDS TERMINATED BY ',' ENCLOSED BY '"';

Далее режем на отдельные файлы:

awk 'BEGIN {FS="\",\"";RS="\"\n"} {peer=$3; gsub(/\/.*|\+/, "", peer); gsub(/"/, "", $1); gsub(/\\\n/, "\n", $4); print $1, $2, $4 >> peer}' /tmp/USER.csv