one of the performance tuning techiques is join with database kept to a minimum. why?

Performing join at database will improve the performance a lot.

take a scenario of joining between the two tables.

Usally we take two database components and place a join component to perform a join between them.This approach takes 2 hits to the data base to retrive the data and perform a join.

Instead of doing in this way we can write a query to perform a join  at the database level by specifing the query while selecting the table. This approach takes only one hit to the data base. 

Join with Database (Join with DB) should not be used if number of records to bejoined with a table is very high as it degradesperformance. Better result can be achieved by retrieving the data from db table and using join component.

There should definitely be an index on the column for the where clause used in the join with db component.