通过Perl脚本从Postgresql数据库表中选择时输出不同

I face quite a strange situation. Via perl script i am connecting to Postgresql database and i am selecting particular columns from concrete table. The output of the SELECT query from DB is saved into File. When the script is executed second time, content of the file from the first select is moved to another file and the actual file is filled with the new select output. Then i am comparing those two files. They should be same at the time of twice script execution. Problem is, that when i run script twice to compare, they differ in one row, when i execute the script third time, the output is the same as it was in frist time and when i execute the forth time, the output is the same as was in second time, and these outputs repeatedly alternates around. I tried 4 different methods how to fetch selected rows from table into file, but still the same situation, however, the content of the table in database is still the same.I face quite a strange situation. Via perl scri