A foreign key in YSQL is used to maintain the referential integrity of data between two tables: values in columns in one table equal the values in columns in another table.
Example:
yugabyte=# CREATE TABLE dim (dim_id INT PRIMARY KEY, some_data VARCHAR);
CREATE TABLE
yugabyte=# CREATE TABLE fact (fact_id INT PRIMARY KEY, dim_id_some_data INT REFERENCES dim(dim_id));
CREATE TABLE
yugabyte=# INSERT INTO dim (dim_id, some_data) VALUES (1, 'A'), (2, 'B'), (3, 'C');
INSERT 0 3
yugabyte=# INSERT INTO fact (fact_id, dim_id_some_data) VALUES (1, 1);
INSERT 0 1
yugabyte=# INSERT INTO fact (fact_id, dim_id_some_data) VALUES (2, 4);
ERROR: insert or update on table "fact" violates foreign key constraint "fact_dim_id_some_data_fkey"
There may be times when you might want to disable a foreign key constraint.
For example, during a migration or data load, you might be loading tables out of order (child table before the parent table).
You can temporaily disable foreign keys in a session while you load the data, then reanble them after the load completes.
Example:
yugabyte=# SET session_replication_role = 'replica';
SET
yugabyte=# INSERT INTO fact (fact_id, dim_id_some_data) VALUES (2, 4);
INSERT 0 1
yugabyte=# INSERT INTO dim (dim_id, some_data) VALUES (4, 'D');
INSERT 0 1
yugabyte=# SET session_replication_role = 'origin';
SET
Have Fun!