how to make query - Mailing list pgsql-sql

we are using Python Django and Postgresql. Our requirement is to search 10 Million Card data and bring specific records as a result after checking the record every time.

We have to find data from a 2nd table that is related to the first one which we are checking using the loop.

For Ex:- 

10000 data - loop 10000 times 

to find data from the 2nd table. 

This makes our query too slower and it gets time out.