As per my knowledge, Informatica will load the data in the default order in which it found in the input file unless you use a primary key/surrogate key in your mapping.
If the order of rows getting loaded in the table is getting jumbled even though you don't use any key, I suggest the following :
Option 1 :
Add a sequence generator in your mapping which generates a sequence number for the dataset. Add the sequence number along with the dataset while loading into the target table.
Create a surrogate key in your target table to insert/maintain the rows in the same order as present in the source flat file.
Hi Krishna, Thank you.