how to fix “OperationalError: (psycopg2.OperationalError) server closed the connection unexpectedly”

Same logic for sqlalchemy.orm, ( on which flask_sqlalchemy is based btw )

engine = sqlalchemy.create_engine(connection_string, pool_pre_ping=True)

More protection strategies can be setup such as it is described in the doc: https://docs.sqlalchemy.org/en/13/core/pooling.html#disconnect-handling-pessimistic

For example, here is my engine instantiation:

engine = sqlalchemy.create_engine(connection_string,
                                      pool_size=10,
                                      max_overflow=2,
                                      pool_recycle=300,
                                      pool_pre_ping=True,
                                      pool_use_lifo=True)

sqlalchemy.orm.sessionmaker(bind=engine, query_cls=RetryingQuery)

For RetryingQuery code, cf: Retry failed sqlalchemy queries