Actually i am getting a csv document from sftp to the staging table i am writing expression rule to count the data. so here i am getting a error memory threshold reached during output conversion. Can some one give perfect resolution for these.

The perfect solution ... without seen your code ... tricky!!!

It seems like you are loading too much data. Try to reduce it.

Depends if you need the number to be displayed in your Appian application. If you are just interested about the total count, 
a SQL statement will do the job:

SELECT COUNT(column_name) FROM table_name