Diagnostics

// A custom DiagnosticsListener SPI implementation
class MyDiagnosticsListener extends DefaultDiagnosticsListener {
    // Override methods here
}

// Configuration is configured with the target DataSource, SQLDialect, etc. for instance Oracle.
try (Connection c = DSL.using(configuration.derive(new MyDiagnosticsListener()))
                       .diagnosticsConnection();
     Statement s = c.createStatement()) {

    // The tooManyRowsFetched() event is triggered.
    // --------------------------------------------
    // This logic does not consume the entire ResultSet. There is more than one row
    // ready to be fetched into the client, but the client only fetches one row.
    try (ResultSet rs = s.executeQuery("SELECT id, title FROM book WHERE id > 1")) {
        if (rs.next())
            System.out.println("ID: " + rs.getInt(1) + ", title: " + rs.getInt(2));
    }

    // The duplicateStatements() event is triggered.
    // ---------------------------------------------
    // The statement is the same as the previous one, apart from a different "bind variable".
    // Unfortunately, no actual bind variables were used, which may
    // 1) hint at a SQL injection risk
    // 2) can cause a lot of pressure / contention on execution plan caches and SQL parsers
    //
    // The tooManyColumnsFetched() event is triggered.
    // -----------------------------------------------
    // When iterating the ResultSet, we're actually only ever reading the TITLE column, never
    // the ID column. This means we probably should not have projected it in the first place
    try (ResultSet rs = s.executeQuery("SELECT id, title FROM book WHERE id > 2")) {
        while (rs.next())
            System.out.println("Title: " + rs.getString(2));
    }
}