I have a database tables which contains two tables with records > 30000.

I wrote a database query
"SELECT rank, b.isbn, book_name, volume_no, author, publisher, price, quantity FROM tbl_rank_books b LEFT OUTER JOIN (SELECT * FROM tbl_rank_stock WHERE customer = " + ddlCustomer.SelectedValue + ") s ON b.isbn = s.isbn AND b.category_id = s.category_id WHERE b.category_id = '" + ddlCategory.SelectedValue + "' ORDER BY rank;"

It took me about 1 minute per time in my developer machine while it has timeout error in the server.

I would like to know if there is any room to optimize the query performance. I have already used indexing to do this. If there are no problems, how can i eradicate the timeout problem?

Thank you