may i know recommended approach to migrate the tables from sql server to postgresql instead of using 3rd party tools
Answering this specifically.
There are none. The community PostgreSQL project maintains no tools that speak to SQL Server.
One possible option to use the PostgreSQL's FDW capabilities with a suitable third-party extension that does talk to SQL Server.
The other is to dump plain SQL, hopefully standard-conforming, INSERT statements from the SQL Server and load them into PostgreSQL via psql.
I suppose SQL Server has some means to export in TSV/CSV or other row-oriented structured output. The \copy psql meta-command or COPY SQL command could load that data.
Depending on the table structures having SQL Server dump the DDL (i.e., CREATE TABLE) for the tables and loading them via psql might be possible as well.
One area that the commercial space helps fulfill is inter-vendor inter-operability tools. While this is a boon to the PostgreSQL community as matter of practicality support for those tools. Before using said tools you should ensure that you can reliably connect, using the standard tools, to the databases in question and perform general actions thereon. Failures there should be resolved first and the communities can help you with that.
