28 December 2020

By default, when Oracle JDBC driver executes query, it retrieves a result set of 10 rows at a time from the database cursor. Low fetch size value might cause more roundtrips to DB and this leads to a longer time to fetch results from queries. You can change the number of rows retrieved with each trip to the database cursor by changing the row fetch size value.

Statement, PreparedStatement, CallableStatement, and ResultSet provides next methods for dealing with fetch size:

void setFetchSize(int rows) throws SQLException

int getFetchSize() throws SQLException

Default fetch size value can be changed by defaultRowPrefetch connection property:

On Wildfly Application Server DataSource level by:

[standalone@localhost:9990 /] /subsystem=datasources/data-source=ExampleOraDS/connection-properties=defaultRowPrefetch:add(value=1000)

On Hibernate level by hibernate.jdbc.fetch_size property:

  <property name="hibernate.jdbc.fetch_size" value="1000" />

I did simple test:

public void defaultRowPrefetchTest() throws Exception {
   EntityManager em = Persistence.createEntityManagerFactory("myDSTestOra").createEntityManager();

   Long time = System.currentTimeMillis();

   Query q = em.createNativeQuery("SELECT * FROM MY_TABLE", Tuple.class);
   List<Tuple> resultList = q.getResultList();

   System.out.println(System.currentTimeMillis() - time);

And on my laptop, fetching of 16K records takes ~185 ms with default value and ~86 ms with defaultRowPrefetch = 20000. As you can see from the result - there is more than x2 performance improvement.

Source code of test case on GitHub