What r the tasks that source qualifier performs?

Join data originating from the same sourcedatabase. You can join two or more tables with primary-foreign key relationships by linking the sources to one Source Qualifier.
Filterrecords when the Informatica Server reads source data. If you include a filter condition, the InformaticaServer adds a WHERE clause to the default query.
Specify an outer join rather than the default inner join. If you include a user-defined join, the Informatica Server replaces the join information specified by the metadata in the SQL query.Specify sorted ports. If you specify a number for sorted ports, the Informatica Server adds an ORDER BY clause to the default SQL query.
Select only distinct values from the source. If you choose Select Distinct, the Informatica
Server adds a SELECT DISTINCT statement to the default SQL query.
Create a custom query to issue a special SELECT statement for the Informatica Server to read source data. For example, you might use a custom query to perform aggregate
calculations or execute a stored procedure.