If your table does not have a primary key it can result in performance issues. This includes long table extract times.
Connector - PostgresSQL Database
The best solution is to add a primary key to the table. You can do this by running this query:
ALTER TABLE table_name ADD PRIMARY KEY (column_1, column_2);
If this is a partition table, the primary key should be globally-unique.
Without a primary key, Fivetran will be forced to read the entire table every time.